could an increase in Bcrossover interference distance^ play a central role?

14 Pages·2016·0.96 MB·English

Chromosoma(2016)125:287–300
DOI10.1007/s00412-015-0571-4
REVIEW
The challenge of evolving stable polyploidy: could an increase
B ^
in crossover interference distance play a central role?
KirstenBomblies1&GarethJones2&ChrisFranklin3&DeniseZickler4&NancyKleckner5
Received:10June2015/Revised:20December2015/Accepted:28December2015/Publishedonline:12January2016
#TheAuthor(s)2016.ThisarticleispublishedwithopenaccessatSpringerlink.com
Abstract Whole genome duplication is a prominent feature andfullyevolvedautotetraploidssuggestthatthesechallenges
of many highly evolved organisms, especially plants. When aresolvedbyspecificrestrictionsonthepositionsofcrossover
duplications occur within species, they yield genomes com- recombination events and, thus, the positions of chiasmata,
prisingmultipleidenticalorverysimilarcopiesofeachchro- whichgovernthesegregationofhomologsatthefirstmeiotic
mosome(Bautopolyploids^).Suchgenomesfacespecialchal- division.Weproposethatacriticalfeatureintheevolutionof
lenges during meiosis, the specialized cellular program that these more effective chiasma patterns is an increase in the
underlies gamete formation for sexual reproduction. effective distance of meiotic crossover interference, which
Comparisons between newly formed (neo)-autotetraploids plays a central role in crossover positioning. We discuss the
findingsinseveralorganisms,includingtherecentidentifica-
ThisarticleispartofaSpecialIssueonBRecentadvancesinmeiotic tion of relevant genes in Arabidopsis arenosa, that support
chromosomestructure,recombinationandsegregation^editedbyMarco thishypothesis.
Barchi,PaulaCohenandScottKeeney.
Electronicsupplementarymaterial Theonlineversionofthisarticle Keywords Polyploidy.Meiosis .Crossoverinterference .
(doi:10.1007/s00412-015-0571-4)containssupplementarymaterial, Homologouschromosomes .Recombination .Chiasmata
whichisavailabletoauthorizedusers.
* NancyKleckner
Introduction
[email protected]
KirstenBomblies
Mosteukaryoticorganismshavediploidgenomes.However,
[email protected]
insomecases,agenomecontainsmorethantwohomologous
ChrisFranklin
copies of each chromosome. This condition, known as
[email protected]
Bpolyploidy,^occursinmanyspecies,notablyplantsbutalso
DeniseZickler manyanimals(Astaurov1969;RamseyandSchemske1998;
[email protected]
OttoandWhitton2000;RamseyandSchemske2002;Comai
2005; Soltis et al. 2007; Doyle et al. 2008; Grandont et al.
1 DepartmentofCellandDevelopmentalBiology,JohnInnesCentre, 2013; Stenberg and Saura 2013; Ianzini et al. 2009).
NorwichResearchPark,Colney,NorwichNR47UH,UK Polyploidy is a potent evolutionary force that is implicated
2 TheRedHouse,St.David’sStreet,Presteigne,Powys(Wales)LD8 inincreasesofgenomecomplexity,adaptation,andspeciation
2BP,UK (Soltis et al. 2003; Rieseberg and Willis 2007; Fawcett and
3 SchoolofBiosciences,UniversityofBirmingham,Edgbaston, VandePeer2010;ArrigoandBarker2012).However,when
BirminghamB152TT,UK thepolyploidconditionfirstarises,itcausessubstantialprob-
4 InstitutdeGénétiqueetMicrobiologie,I2BC,UniversitéParis-Sud, lems for basic cellular processes, most notably for regular
Orsay,France chromosomesegregationduringmeiosis,thespecializedcel-
5 DepartmentofMolecularandCellularBiology,HarvardUniversity, lular program that underlies gamete formation for sexual re-
Cambridge,MA,USA production (Ramsey and Schemske 2002; Comai2005;
288 Chromosoma(2016)125:287–300
StenbergandSaura2013;below).Nonetheless,thisproblem Comai 2005; Grandont et al. 2013; Sybenga 1975).
canultimatelyberesolved:stable,sexuallyreproducingpoly- However,naturallyoccurringpolyploidscanevolvesolutions
ploidlineagesarefoundinnatureandsometimescanbegen- toovercometheseproblems.
erated in experimental settings upon selection for fertility.
How these states evolve remains largelymysterious, and in-
vestigation of this question promises intriguing insights into Diploidmeiosis
how fundamental processes can be evolutionarily modified
withoutperturbingtheiressentialfunctions. Tocontextualizethechallengesfacedbypolyploidsinmeio-
Polyploids generally fall into two broad categories, albeit sis,wefirstdiscussthenormalprocessthatoccursindiploids.
with a range of intermediate states (Ramsey and Schemske InadiploidcellatmetaphaseI,thecentromeresofhomol-
1998; Otto and Whitton 2000; Bomblies and Madlung ogous chromosomes (homologs) are oriented towards oppo-
2014).BAutopolyploids^arisebywholegenomeduplication site poles, in preparation for segregation during the ensuing
within an individual or within a species, yielding genome anaphase(Fig.1(a,b)).Atthisstage,homologsareconnected
complementsinwhichallcopiesofaparticularchromosome tooneanotheratspecificpositions,seencytologicallyasone
arecloselyhomologous(althoughstillcarryingsequencedif- or more Bchiasmata,^ each of which results from the com-
ferencespresentbetweenhomologsintheoriginatingdiploid). bined effects of a DNA crossover (CO) between homolog
BAllopolyploids,^ in contrast, arise by formation of hybrids non-sister chromatids plus links between sister chromatid
withrelatedyetdistinctgenomes,accompaniedbywholege- armsalongtheirlengths(Fig.1(a,b)).
nomeduplication.Thecomponentgenomesofallopolyploids Theseconnectionsareessentialfortheregularsegregation
thus comprise two or more sets of diploid genomes, with a ofhomologstooppositepoles,analogoustothewayinwhich
closer homology between the two chromosomes of each set aphysicalconnectionofsisterchromatidsensuresbipolarori-
and a significantly less homology between chromosomes of entation and segregation during mitosis and metaphase/
differentsets. anaphase of the second meiotic division (Li and Nicklas
In diploid meiosis, homologs regularly segregate away 1997;Nicklas1997).Inallcases,connectednessisimportant
from one another, to opposite poles, at the first division of because it allows centromere/kinetochore complexes to be
meiosis (MI); sister chromatids then segregate at the second placed under mechanical tension as the spindle is forming.
divisionofmeiosis(MII).Forfullfertilityinpolyploidmeio- Thistension,inturn,hastwo effectswhich,together,ensure
sis,theMImuststillsomehowresultinaregularsegregation bipolar orientation of the segregating units (Li and Nicklas
ofequalnumbersofparentalchromosomestoeachpole,de- 1997; Nicklas 1997; Lampson and Cheeseman 2011). First,
spitethefactthattherearenowmultiplecopiesofeachchro- tension stabilizes microtubule/kinetochore attachments, thus
mosomepresent.Thischallengeisparticularlystarkforauto- locking eachbivalentintothe correctconfiguration.Second,
polyploids, because the different versions of each chromo- tension signals to the spindle regulatory surveillance system
someareverysimilarorindistinguishableinDNAsequence (theBspindleassemblycheckpoint^)thatabivalentisproperly
and thus provide no special intrinsic cues to guide equi- alignedandthusreadytobecorrectlypulledtowardsopposite
partitioning during the segregation process. For allopoly- poles during anaphase I. When all segregating pairs (e.g.,
ploids,incontrast,thepresenceofhomologycanbe(andis) meiotichomologBbivalents^)arecorrectlyorientedtooppo-
used to solve this problem, as shown by the fact that more site poles, cellular regulatory mechanisms trigger anaphase
homologouschromosomespreferentiallysegregatefromone onset.Intheabsenceofcorrectorientation,anaphaseeventu-
another(e.g.,Holm1986;discussioninZicklerandKleckner allyproceedsbutwithasignificantdelay.
1999;Bhullaretal.2014;Martínetal.2014).Wefocushere Inaccordancewiththeseconsiderations,afirstrequirement
onthemoredramaticcaseofautopolyploidy,whichhasbeen forregularMIsegregationisthateachpairofhomologsmust
investigatedextensivelyfromvariousperspectives. (and does) acquire at least one CO (and thus chiasma). This
feature is often called the Bobligatory CO.^ Many diploid
species exhibit only one or two COs/chiasmata per bivalent
Newlyarisingautotetraploidsevolvetoastable (e.g.,ArabidopsisthalianaandArabidopsisarenosa;Fig.1(a,
fertilestateviarestrictionsonthenumberandtypes b));othersexhibitlargernumbers.
ofchiasmaconfigurations A second requirement for regular MI segregation is that
eachbivalentmustbea separatephysicalunit thatisfree of
Newlyemergedautopolyploidlinesgeneratedinthelaborato- entanglements with other bivalents. In fact, interlockings
ryorarisinginnaturegenerallyexhibitmeioticchromosome among unrelated chromosomes often arise during prophase
mis-segregation in meiosis I, resulting in the formation of as a consequence of the way in which homologs undergo
aneuploid gametes and compromised fertility (Ramsey and recombination while becoming coaligned/paired and syn-
Schemske 1998, 2002; Hilpert 1957; Santos et al. 2003; apsed. However, interlocks are also concomitantly actively
Chromosoma(2016)125:287–300 289
A. thaliana A. arenosa
A diploid
1
3
5
4
2
5S rDNA
45S rDNA chrs 1-5 5 µm
B
II1 II1 II5 II2
II5
II2
C new autotetraploid
U
II2 IV3 IV1 E
E E
IV4
IV5 II2
E
II=bivalent IV=quadrivalent E=entanglement U=univalent
Fig. 1 Metaphase I (MI) configurations in diploids and newlyarisen configurations seen in a (left). The arrows indicate the orientation of
autotetraploidsofArabidopsis.Chromosomesarebeingpulledtowards centromeres (filled circles) towards opposite spindle poles. II denotes
oppositespindlepoles(aboveandbelow,respectively)viaattachmentsof thebivalent;superscriptdenotesthechromosomenumber. c(Left)An
microtubulestotherespectivecentromere/kinetochoreregions.a(Left) experimentallycreatedautotetraploidofA.thalianashowingamixtureof
Arabidopsisthalianadiploidshowingfivebivalents(fromLópezetal. bivalents(II)andquadrivalents(IV)(fromSantosetal.2003).c(Right)
2012).Thechromosomenumberofeachbivalentisindicated.45Sand5S Anexperimentally created autotetraploid ofA. arenosa showing some
rDNAlociareindicated.a(Right)Arabidopsisarenosadiploidshowing identifiable bivalents, many complex configurations in which multiple
eightbivalents(C.F.andC.Morgan,unpublished).bCartoonsshowing chromosomesareentangled(E)andoneapparentunivalent(U)(Chris
chromosome associations that give rise to three of the metaphase I MorganandC.F.,unpublished)
eliminated (Storlazzi et al. 2010). Thus, at MI, interlocked alsoformdiversetypesofmultivalents(wheremorethantwo
bivalentsarerareorabsent. copiesareconnected).
Manyoftheconfigurationsthatarise inneo-autotetraploids
Meiosisinneo-autotetraploids areincompatiblewithregulartwo-by-twosegregation.Overall,
the operative rule for effective two-by-two segregation is that
A newly arisen autotetraploid line faces severe challenges everyoneofthefourhomologouschromosomesmustbelinked
duringMIchromosomesegregation.Sinceallfourcopiesof toatleastone,butnotmorethantwo,partnerchromosome(s)
eachchromosomeareessentiallythesame,COscan(anddo) (Fig.2).Setsoflinkagesthatsatisfythisrulecomprisebivalents
occur indiscriminately among these copies in all pairwise andeitherchainorringquadrivalents(Fig.2a).Inallthreecases,
combinations.Thissituationcompromisesbothofthefeatures spindle tension is achieved by orientation of two centromere/
requiredforregulartwo-by-twosegregation. kinetochore regions towards each pole as appropriate to two-
First,newlyarisingautotetraploidsexhibitcomplexarrays by-two segregation. In contrast, any complement that includes
ofchiasmaconfigurations(e.g.,Fig.1(c)).Thefourchromo- a chromosome(s) unlinked to any partner will be ineffective
somecopiesmay happento form two pairs ofbivalents that (e.g., a univalent-plus-trivalent combination; Fig. 2b, left).
willgiveregulartwo-by-twosegregation.However,theycan Additionally, in any complement where one or more
290 Chromosoma(2016)125:287–300
Auto-tetraploid Chiasma Configurations
A Effective
adjacent alternate
B Not Effective
Fig. 2 Chiasma configurations for an autotetraploid that are either requirement. By contrast, if any chromosome (or more than one
effective for ensuring two-by-two segregation (a) ornot (b). Effective chromosome) is unlinked to a partner or is linked to all three other
segregation requires that each chromosome be linked to either one or homologs,segregationwillbeaberrant,asillustratedforrepresentative
two other chromosomes. Only three configurations satisfy this singlechromosomecases
chromosomesarelinkedtoeachofthethreeotherhomologous 1989;Wolfetal.1989;Fjellstrometal.2001;Hollisteretal.
chromosomes, one centromere will always be unable to come 2012):eachlocussegregatesfouralleles,withtwoofthefour
under tension and thus again will not reliably segregate to the parentalallelesrandomlysegregatingintoeachgamete.Thus,
appropriatepole(Fig.2b,right). evolutionofanautotetraploidintoastable,sexuallyreproduc-
Second,neo-autotetraploidchromosomesexhibitextensive ingstatehasachievedregulartwo-by-twosegregationwithout
numbersofchromosomalassociationsthatappeartobecom- the emergence of specific genetically defined partner prefer-
prised of both multivalents and entanglements (e.g., BE^ in ences.Todoso,autotetraploidevolutionincludeschangesin
Fig.1(c);seealsoFig.5inYantetal.2013).Theseconfigu- chiasmapatterns.
rations are the consequence of the fact that all four homolo- First,there is a reductioninthe total overall frequency of
gouschromosomesoftenpairandsynapseandformCOcon- chiasmata. In general, well-evolved autotetraploids tend to
nections; moreover, if other homologs are trapped in those have lower overall frequencies of COs than newly formed
quadrivalents, they can remain entangled by their own CO autotetraploids (e.g., Morrison and Rajhathy 1960a;
connections up to metaphase I. In diploids, such entangle- Mulligan 1967; Reddi1970 ; Yant et al. 2013; Wu et al.
ments (Binterlocks^) are usually resolved before diplotene/ 2013).Morespecifically,stableautotetraploidlinesthathave
metaphaseI(vonWettsteinetal.1984).However,iftheyare evolvedinnaturehavelowerCOfrequenciesthanthediploid
not,thepresenceofentanglementsisanobviousproblemfor linesfromwhichtheyoriginated(Mulligan1967;Yantetal.
MIsegregation.Innewautotetraploids,thepresenceandper- 2013).Furthermore,whenanautotetraploidiscreatedexper-
sistence of interlockings might primarily be responsible for imentallyandthenallowedtoevolveunderselectivepressure
the overall high mis-segregation frequencies as well for the forsuccessfulmeiotictransmission,adecreaseinmultivalent
occurrence of univalents. The resolution of interlocks re- frequency commonly occurs, associated with a reduction in
quires,amongotherevents,Mlh1-mediatedreleaseoftrapped the level of COs, which progressively decreases with the
recombinational interactions (Storlazzi et al. 2010). Such a number of rounds of selection (e.g., Bremer and Bremer-
processwouldpromotedisassemblyofCO-fatedrecombina- Reinders 1954; Povilaitis and Boyes 1956; Hilpert 1957;
tioncomplexesthatarestalledatsitesofinterlocks.Giventhat Morrison and Rajhathy 1960a; Lavania et al. 1991; Santos
CO complexes are relatively few to begin with, the result et al. 2003). In a number of species, the frequency of chias-
wouldbeatendencyforsomeoftheentrappedchromosomes mata is reduced to one, thus nearly the minimum possible
toloseallCOs,thuslackingeventhesingleBobligatory^CO level. For example, autotetraploids Lotus corniculatus,
andappearingasunivalent(s)atMI. A. arenosa, andPhysaria vitulifera all average about 1.1
crossovers per bivalent (Mulligan 1967; Davies et al. 1990;
Meiosisinevolvedautotetraploids Yantetal.2013).LowernumbersofCOs/chiasmatanotonly
match the emergence of more restricted chiasma configura-
Evolved autotetraploid species commonly show regular tions (above) but also will tend to reduce the probability of
tetrasomic inheritance (Quiros 1982; Krebs and Hancock persistingMIinterlockings.
Chromosoma(2016)125:287–300 291
Theonlyexceptiontothistrendisthat,inthefewrelated A. arenosa evolved autotetraploids
grass species where autotetraploids exhibit mostly quadriva-
lents with terminal chiasmata (e.g., Dactylis glomerata; A 5 pairs of bivalents
Table S1), the chiasma frequency is higher than that in the + 3 quadrivalents
corresponding diploids or newly formed autotetraploids.
Apparently,anevolvedautotetraploidstatewiththreeorfour
chiasmata per quadrivalent emerged from a standard diploid
conditionwithoneortwochiasmataperbivalent.
Second, MI chiasma configuration types become more re-
stricted,beinglimitedspecificallytothosethateffectivelypro-
mote regular two-by-two segregation. Thus, trivalent-plus-
univalentcombinationsarerareinnaturallyevolvedtetraploids
45S rDNA
and,inexperimentalevolutionstudies,occurwithadecreasing 5S rDNA
frequencyasastablestateisprogressivelyachieved(Povilaitis
bivalents quadrivalents
and Boyes 1956; McCollum 1958; Hazarika and Rees 1967; 1a1b 2a 2b 3a 3b
7ab 8ab
Jones 1967; Santos et al. 2003). Furthermore, a survey of 20 4ab
naturally evolved autopolyploid species (Table S1) reveals 12
speciesthatformexclusivelyoralmostexclusivelybivalentsat 5a5b 6a 6b
MI, with each set of four chromosomes comprising a pair of
bivalents; five other species show mostly bivalents, but also
chain rings(adjacent)
somequadrivalents(e.g.,A.arenosa;Fig.3a,b),andthreere-
latedspeciesshowprimarilyorexclusivelyquadrivalents.The
bivalent-plus-quadrivalentsolutionalsoemergesinthelaborato- B 16 bivalents (8 pairs)
ry when a newly formed autopolyploid evolves into a more
stablelinebyselectionforfertilityoveranumberofgenerations,
withimprovementssometimesseenafteronlyafewgenerations
of selection (Gilles and Randolph 1951; Bremer and Bremer-
Reinders1954;Hilpert1957;Santosetal.2003).Moreover,the
quadrivalentsseenintheseevolvedsituationsareeitherchainsor
rings(Fig.3b),i.e.,thetwospecifictypesthatareeffectivefor
two-by-two segregation (Fig. 2a). In contrast, other possible Fig.3 TwometaphaseIcomplementsforafullyevolvedautotetraploid
quadrivalent configurations are rare, absent, and/or anti- ofArabidopsisarenosa(C.F.andC.Morgan,unpublished).aAmajority
ofbivalentsplusaminoritychainandringquadrivalents(10bivalents
correlatedwithregularsegregation(e.g.,McCollum1958).
corresponding to 5 pairs plus 3 quadrivalents). Accompanying color-
Ring quadrivalents can exhibit either of two MI segrega-
inverted images show chromosome constitution and multivalent
tion configurations according to whether Balternate^ or configurations. b Full complement of 16bivalents corresponding to 8
Badjacent^centromeresarelinkedtothesamepole(Figs.2a pairs
and3a).Theformerconfigurationisfavored:asautotetraploi-
dy evolves, the alternating ring configuration increases in
abundance while the adjacent ring configuration decreases CO-correlated Mlh1 foci along prophase chromosomes
(McCollum 1958; Mosquin 1967). These preferences corre- (ChrisMorgan,C. F.,and K.B.,unpublished). Onthe other
spondtothedictatethatspindletensionshouldbemaximized: hand, the occurrence of chiasmata near chromosome ends
in rings of the favored alternate orientation, all four (referredtoasBterminal^localization)ispositivelycorrelated
centromere/kinetochore complexes are under tension from with regular quadrivalent segregation (Myers 1945;
both sides whereas, in the less-favored adjacent orientation, McCollum1958;HazarikaandRees1967;Jones1967),im-
pairs of bi-oriented complexes are under tension from only plyingthatsuchlocalizationmightbeparticularlyhelpfulfor
oneside. creatingtheappropriatequadrivalentconfigurations.Inanoth-
Third,thereisatendencyformodulationofchiasmaposi- er variation, some autotetraploid Allium species have
tion during autotetraploid evolution. There is no universal centromere-proximalCOs/chiasmata(TableS1).
requirement for localization of chiasmata to particular posi- Chiasmata are prominently terminal in autotetraploids of
tions.Forexample,inA.arenosa,thechiasmatainanevolved grasses and cereals (Hazarika and Rees 1967; McCollum
autotetraploidcanoccuratcentromere-proximal,centromere- 1958).However,thesametendencyisalsoseeninthecorre-
distal(Bsub-terminal^),andinterstitialpositionsasseenfrom spondingdiploidlines.Perhapsearlierintheirevolution,dip-
MI configurations and confirmed by molecular analysis of loids became autotetraploids, which evolved terminal
292 Chromosoma(2016)125:287–300
chiasmata,andthenreturnedtothediploidstate.Indeed,many explain chiasma patterns in a variety of diploid species
apparently diploid genomes give evidence of prior (Zhang et al. 2014a, b; Wang et al. 2015), has two key fea-
polyploidization (e.g., Mitchell-Olds and Clauss 2002). tures. First, each CO designation sets up an Binterference
More generally, terminal localization of chiasmata might fa- signal^thatspreadsoutwardinbothdirectionsfromthedes-
cilitate ready interconversion between diploidy and ignationsite,disfavoringtheoccurrenceofadditionalCOsin
autotetraploidy. its path. This signal is strongest at its point of origin and
Fourth,evolvedautotetraploidsalsolackthehighlevelsof dissipatesin strengthwithan increasing distance away from
interlockingsamongunrelatedchromosomesthatcharacterize thatstartingposition.Thus,afirstCOdesignationwillsetupa
newlyemerged lines,withresolutionduringpachytene asin surrounding zone of interference. A second CO designation
thediploidcase(compareFig.3a,bversusFig.1(c);Higgins willtendtooccuroutsidethatzoneofinterference.Anysub-
etal.2014a;Yantetal.2013). sequentCOswilltendtoBfillintheholes^betweenthepre-
viouslyestablishedzones,resultinginatendencyforCOsto
beevenlyspaced,asobserved(e.g.,Fig.4e).
ModulationofCOformationforautotetraploid Second,theprocessofCOdesignationisveryefficient.As
evolution one consequence of this effect, each pair of homologs will
undergo at least one CO designation, thus ensuring a first
Whattypeofmechanism(s)mightexplainhownewlyformed obligatory CO. Thereafter, CO designations will continue to
autotetraploids evolve the specific chiasma configurations occur as long as there are still regions that have not been
neededtosupportregulartwo-by-twoMIsegregation?Since affectedbyinterference(or,moreprecisely,wheretheeffects
CO positions are determined during prophase, evolutionary of interference are not great enough to impede CO designa-
forcesarepresumablyactingoneventsthatoccurduringthis tion).Given thissituation,thefinalarray ofCOswillbede-
period,longbeforechiasmataareactuallyrequiredtomediate terminedbythreefactors:theposition(s)ofearlyrecombina-
chromosomealignmentandsegregation. tion interactions along the chromosomes, the strength of the
COdesignationprocessinrelationtotheinhibitoryeffectsof
COformationindiploidmeiosis interference, and the distance over which CO interference
acts. In an extreme case, where the interference distance is
Universally, meiosis involvesthe initiationofrecombination longerthanthelengthofthechromosome,eachhomologpair
via a large number of programmed double-strand breaks willacquire one and onlyone CO, regardlessofthe number
(DSBs) which interact primarily with homolog partners to and positions of early recombination interactions. In diploid
give a large number of early recombinational interactions species, this situation occurs both genome wide (e.g.,
(Hunter2006;ZicklerandKleckner2015).Aminoritysubset Caenorhabditis elegans; Martinez-Perez and Colaiacovo
of these many interactions is then designated to eventually 2009;HillersandVilleneuve2003)andfortheshorterchro-
matureintoCOs(BCOdesignation^)withtheremainderma- mosomes inthe complements ofseveral organisms,e.g., the
turing to other fates. When a bivalent exhibits more than a locustSchistocercagregaria(Fox1973).
single CO, those COs exhibit the classical feature of Inmostorganisms,includingArabidopsisandotherplants,
Bcrossoverinterference^:thepresenceofaCOatoneposition COdesignationandinterferencearethoughttobeimplement-
isaccompaniedbyareducedprobabilitythatanotherCOwill edataparticularstageofmeiosis,lateleptotene,whenhomo-
occurnearby(Sturtevant1915).Thestrengthofthisreduction logs are coaligned at a distance of ∼400 nm (Fig. 4b–d, f;
decreaseswithincreasinginterpositiondistance.Importantly, Zhang et al. 2015; Sanchez Moran et al. 2001; C.F., unpub-
DSB formation and all ensuing DNA events leading to CO lished).Coalignmentisaffectedbyearlyrecombinationinter-
formationoccurinrecombinationcomplexesthatareindirect actions, which, in favorable cases, can be seen as Bbridges^
physicalandfunctionalassociationwithdevelopingordevel- linkingthestructuralaxesofcoalignedpartnerchromosomes
opedaxes(Kleckner2006;Kleckneretal.2011;Zicklerand (Fig.4b–d,f;ZicklerandKleckner2015).Notably,however,
Kleckner2015;Storlazzietal.2010).Correspondingly,itnow the sameconclusions willpertain aslongasCOdesignation
appearsthattheBmetric^fortheinterferenceeffectisphysical and interference operate on the array of total DSB-mediated
distancealongthechromosome,e.g.,alongchromosomeax- recombinationinteractions,whetherspecificallyattheBbridge
es, with absolute distances ranging from 300 nm to many stage^ornot.
micronsaccordingtotheorganism,ratherthaneithergenomic
distance(Mb)orgeneticdistance(cM)(discussioninZhanget COformationinautotetraploids:aproposal
al.2014b).
WehaveproposedaspecificmodelforhowCOpatterning Theabovedescriptionofmeioticprophasewouldsuggestthat
mightoccur(Fig.4a;Kleckneretal.2004;Wangetal.2015 inanewlyformedautotetraploid,DSBswilloccuronallfour
and references therein). This model, which can accurately chromosomes and DSB-mediated bridge interactions will
Chromosoma(2016)125:287–300 293
A CanOd- dInetseirgfnearetinocne hmuamlean B hSuCmsan male E
centromeres
Mlh1 foci (COs)
ns RPA bridges
atio 10µm SC forming
interference sign C
e
d
O
C
interference F Allium cepa
maturation
1µm
CO CO
D
x x chiasmata
SC forming
DSB-mediated inter-homolog 2.5µm
interactions(e.g. bridges)
before interference 1µm Allium cepa
after interference
Fig.4 Prophasechromosomaleventsindiploidmeiosis.aBFill-in-the- mediated interactions) visualized by electron microscopy of PTA-
holes^modelforCOpositionselection(seealsoWangetal.2015).The stainedspreadpreparations(fromAlbiniandJones1987).bLeptotene/
array of early total DSB-mediated recombinational interactions (e.g., zygotene nucleus illustrates bridges containing single-strand binding
bridges;b–d(below))isacteduponbyaCOdesignationprocess.Each protein RPA (white arrows), a directplayer in Rad51/Dmc1-mediated
COdesignation(redstar)setsupaninhibitoryzoneofBCOinterference^ strand exchange for recombination, with accompanying onset of
(blue arrows) via a signal that spreads outwards in both directions, synapsis.GreenindicatesSMC3cohesinaxis,bluecentromeres(blue),
dissipatingwithdistance.Thissignalpreventsbridgeinteractionsinthe and red RPA protein (from Oliver-Bonet et al. 2007). c, d Bridge
affectedregionfromundergoingCOdesignation(indicatedbybridges configurationsandincipientsynapsiscorrespondingtothestageinb.e
changedtoyellow).SubsequentCOdesignationsoccurinregionsaway Pachytene synaptic configurations with SYCP3 axes of the SC (red),
frompreviouslyestablishedinterferencezones,ultimatelyfillinginthe centromeres(blue),andMlh1focimarkingsitesofCOs(green)(from
holesbetweenpreviousCOsites.COdesignationisveryefficient,thus Gruhn et al. 2013). f Two bivalents showing, respectively, extensive
ensuringthatallhomologpairsacquireatleastone(first,obligatory)CO. synapsisinprogressandcoalignment.Inc,d,f,blackarrowsindicate
b–fHomologcoalignment(Bpairing^)ismediatedbyinter-axisbridges examplesofBnodules^orbridgesoffivetypes:(a)associatedwithSCs,
thatcompriseDSB-mediatedrecombinationalinteractionsandfollowed (b) with association sites, (c) midway between axial cores in close
by SC formation (Bsynapsis^). b, e Human male meiotic prophase alignment,(d)pairedstructuresatmatchingsitesonaxialcores,and(e)
chromosomes visualized by immunofluorescence illumination. c, d, f apparentlybridgingthespacebetweentwoconvergingaxialcores.Blue
Allium cepa axes and associated Bzygotene^ recombination nodules arrowsandtextindicatepositionsofforming/formedSC
(ZNs) or bridges (corresponding approximately to many/all DSB-
occurpromiscuouslyamongallpossiblepairsofchromosome COsite.Alternatively,giventhatthemetricofinterferenceis
axes. CO designation would then be imposed on this four- physical chromosome length (above), the equivalent effect
chromosome array. This notion is supported by tetrasomic could be achieved by shortening the physical lengths of the
inheritance (above) and by studies of axis relationships in chromosomes: if chromosome length is decreased, a given
autotetraploids,wherecoalignmentofmultipleaxeshasbeen interference distance will comprise a larger fraction of chro-
observed prior to and after synaptonemal complex (SC) for- mosome length not only in physical distance but also in ge-
mation. Such complex partner interactions are common in nomicdistance(Mb)orgeneticdistance(cM).
newly formed autotetraploids (Jones and Vincent 1994; An increase to an appropriate (effective) interference
Stack and Roelofs 1996; Carvalho et al. 2010; Rasmussen distance will reduce the total number of COs/chiasmata.
1987;JonesandVincent1994;Fig.5a–e). It will also specifically generate exactly the chiasma con-
We suggest herethata criticaleventinthe evolution ofa figurations observed in evolved autotetraploids. The ef-
fertileautotetraploidspeciesisanincreaseintheBinterference fectiveness of this change is a direct consequence of the
distance,^i.e.,theeffectivedistanceoverwhichtheinhibitory basic logic of the CO patterning process (above). The
interferencesignalspreadstothepointwhereitiscomparable efficiency of CO designation will ensure that every chro-
to, or greater than, the total lengths of the chromosomes mosome copy acquires at least one CO/chiasma, thus
(Fig. 6a). This required effect could be achieved by directly eliminating Bzero-CO^ univalents in general and
altering the spreading interference process per se, sothatin- trivalent-plus-univalent configurations in particular.
hibition extends over a greater distance from the nucleating Concomitantly, operation of interference over a
294 Chromosoma(2016)125:287–300
A B
C D
F G
E
Fig. 5 Prophase relationships among homologous chromosomes in (e.g.,arrowind). f, g Two configurations exhibitingnearly complete
autotetraploids. a Coalignment offour homologous chromosome axes synapsis.Thefourchromosomesaredrawnwithdifferentcolors.Note
at mid-prophase in tetraploid onion (Allium porrum) (from Stack and thatchromosomestwistduringSCformation.Quadrivalentfrequencies
Roelofs1996).Thearrowsindicateearlyrecombinationnoduleswhich diminish as the extent of synapsis increases such that, by the end of
markthesitesofearlyrecombinationinteractions.bImmunostainingof pachytene, the frequency of quadrivalents closely matches the
thespreadArabidopsisarenosatetraploidforaxiscomponentASY1and frequency of chiasmata seen at metaphase I. By implication, the
SC component ZYP1, showing both coalignmentof all four homolog associations seen at the end of pachytene are stabilized by the
axes (left arrow) and pairwise synapsis (right arrow) (c) (C. Morgan occurrenceofcrossingover(or,atleast,Bcrossoverdesignation^),with
and C.F., unpublished). c–g Quadrivalents in tetraploid Bombyx one CO on each of the four arms. Once the SC disappears, these
spermatocytes (from Rasmussen 1987). c–e Three examples of pachytenequadrivalentsleadtoringquadrivalentsatmetaphaseI
configurationsexhibitingpartialsynapsispluspre-synapticassociations
sufficiently long distance can ensure that no chromosome homologs.Theresultwouldbeapairofbivalents,eachlinked
becomes connected to more than two partners and, more by only a single CO. In this situation, the COs in question
specifically, that such two-partner connections will occur couldoccuranywherealongthelengthsofthechromosomes.
near the ends of the chromosomes. These effects are illus- In a less extreme case, where the interference distance is
trated below (Fig. 6b, c). somewhatlessthanthelengthofachromosome,thepossible
outcomeswillbebivalents,witheitherasingleinterstitialor
Predictions two terminal chiasmata, and ring and chain quadrivalents
linked by terminal chiasmata (Fig. 6b, c). These outcomes
IfCOinterferenceactsoveradistancelongerthanthelength canbeunderstoodasfollows:
ofaparticularchromosome,afirstCOdesignationwillalways
precludetheoccurrenceofasecondCOdesignationanywhere (1) IfthefirstCOdesignation occursataninterstitialloca-
elseon eitherofthe two participating homologs, eveninre- tion,itwillprecludeCOdesignationsanywhereelseon
gions where they are paired to other partners. A second CO the involvedchromosomepair, thusensuringthatother
designation will then occur only on one of the other two COdesignationsinvolvetheotherpairtocreateapairof
Chromosoma(2016)125:287–300 295
A Hypothesis
interference distance is
comparable to
chromosome length
B Bivalents
(i) (ii)
x + x
x + x x x x + x
x x + x x
C Quadrivalents
(i) (ii)
1
2
3
4
1
1
2 x
3 chain 2
4 x
3
1
x
4
ring 4 2
3
Fig.6 Predictedchiasma/COpatternsforanevolvedautotetraploid.a interstitial(i)orsub-terminal(ii)andaccordingtothestartingarrayof
COsitesareproposedtobeselectedwithefficientCOdesignationand earlyrecombinationalinteractions(notshown).cQuadrivalentscanonly
accompanyingCOinterferencewhichextendsoveraneffectivedistance ariseifthefirstchiasmaissub-terminalandwill compriserings(i)or
comparabletothelength(s)ofthechromosome(s)(text;representations chains (ii) according to the particular starting array of early
asinFig.4a).Iftheinterferencedistanceislongerthanthechromosome recombinational interactions. In the example shown, the right-most
length,thepredictedoutcomeisafullcomplementofbivalents,eachwith interaction between top and bottom chromosomes is present in i but
asinglechiasma(notshown).b,cPredictedoutcomesiftheinterference absent in ii, thereby limiting the number of CO designations to 3.
distanceissomewhatlessthanthetotalchromosomelengthcomprisea Importantly, the occurrence of univalents, e.g., in trivalent-plus-
mixture of bivalents and quadrivalents, where the quadrivalents are univalent configurations, is precluded with efficient CO designation
chainsorringsinwhichallchromosomesarelinkedby(sub-)terminal whichensuresthateverychromosomewillexperienceatleastonesuch
chiasmata.bBivalentsofdifferenttypescanariseifthefirstchiasmais event(text;notshown)
bivalents. The second pair might also acquire a single (2) IfthefirstCOdesignationoccursnearachromosome
interstitial CO; however, if the second CO designation end, diverse patterns can arise according to the loca-
is near a chromosome end, the third CO designation tions of the early recombination interactions and the
could occur at the opposite end, generating a bivalent particular sequence of subsequent CO designations
withapairofterminalCOs(Fig.6b(i)). (Fig. 6b(iii), c). Univalents are excluded by efficient
296 Chromosoma(2016)125:287–300
COdesignation.Thus,theonly possibilitiesarebiva- Wolf et al. 1989; Davies et al. 1990; Carvalho et al. 2010;
lents and quadrivalents. Moreover, since an event at Yantetal.2013).
oneendprecludestheinvolvementofthetwoaffected The proposed mechanism predictsthatwithin a given or-
chromosomesalongmostoftheirlengths,anysecond ganism,whendifferentchromosomesareofdifferentlengths,
CO designation involving one or both chromosomes bivalentswillbemorelikelyforshorterchromosomeswhere-
oftheoriginalpairwilloccurattheoppositeendfrom asquadrivalentswillbemorelikelyforlongerchromosomes.
the first CO designation. Given these rules, the only Correspondingly,inexperimentallyevolvedautotetraploidsof
possibleoutcomeisapairofbivalents[atleastoneof A. thaliana and natural polyploids of Zea perennis and
which has a pair of terminal chiasmata, and the other L. corniculatus, the shorter chromosomes tend to occur as
ofwhichmaybethesameormayhaveasingleinter- bivalents whereas the longer chromosomes tend to occur as
stitialchiasma(Fig.6b(ii));aslinkagesprogressfrom quadrivalents with terminal associations (Dawson 1941;
chromosome to chromosome, ring and chain quadri- Shaver 1962; Davies et al. 1990; Santos et al. 2003).
valents linked by terminal chiasmata (Fig. 6c)]. Interestingly,also,whenchiasmaconfigurationsareanalyzed
Chains and rings are distinguished by the particular asafunctionofevolutionarytimeafteradenovocreationof
patternofearlyrecombinationeventslinkingthefour an autotetraploid, regular segregation is achieved first for
chromosomes prior to CO designation (Fig. 6c(i, ii)). shorterchromosomeswithlongerchromosomesfollowinglat-
er(Santosetal.2003).
Itcanbenotedthattheseoutcomescanemergefromsitu- One way to increase the effective interference distance
ationsinwhichthefourhomologouschromosomesarelinked would be to decrease chromosome length (above). In accor-
by early recombinational interactions all along their lengths. dancewiththispossibility,evolvedautotetraploidsinthethree
Ofcourse,however,ifearlyCOdesignationsoccurpreferen- well-definedcaseshavebeenshowntohaveshorterprophase
tially near the chromosome ends, then outcomes involving chromosomesthanthediploidsfromwhichtheyevolved.This
terminal chiasmata will be more strongly favored. A recent is true in the sand cress (A. arenosa; Higgins et al. 2014a),
analysis of diploid barley raises an interesting possibility of alfalfa (Medicagosativa; Gillies 1969),and malesilkmoths
thisnature(Higginsetal.2014b).Inthatspecies,theinitiation (Bombyx mori; Rasmussen 1987). In Bombyx, for example,
of recombination occurs along the lengths of the chromo- the total diploid SC length per nucleus is 213 μm at early
somes but occurs much earlier near chromosome ends than pachyteneand215μmatlatepachytenewhilethetetraploid
ininterstitialregions.Moreover,COs/chiasmataoccurdiffer- lengthis190μmatearlypachyteneand186μmatlatepachy-
entially in the terminal regions. It was suggested that this tene. Whether the actual interference distance (in μm) also
pattern could reflect the operation of CO interference, with increasesinthesesituationsremainstobedetermined.
earlyCOdesignationsinterminalregionssettingupinterfer- Incerealsandgrasses,allCOs/chiasmataarerelativelyter-
encethatspreadsinward,thusprecludinginterstitialCOdes- minal. Classical studies have argued for a model in which
ignations.Importantly,bythishypothesis,terminalchiasmata pairs of bivalent Bends^ engage in CO formation randomly
aretheindirectconsequenceofthreefactors:localizedrecom- in all combinations (the so-called Brandom end pairing
binationinitiation, the temporalprogram ofCOdesignation, model,^wherethewordBpairing^wasusedtodenotechias-
and CO interference over an appropriate distance. There maformation;MorrisonandRajhathy1960a,b;discussionin
wouldbenointrinsic localpreferenceforCOdesignationto Santosetal.2003).Thepredictedproportionofbivalentsver-
occuratchromosomeendsperse.Operationofsuchasystem sus quadrivalents in such case is ∼1/3 versus ∼2/3, which
wouldobviouslybeadvantageousalsoforautotetraploids. matches the experimental observations in these organisms
We alsonotethata rolefor interference for chiasma con- (e.g., Morrison and Rajhathy 1960b). This outcome is ex-
figurationsintetraploidshasbeenconsideredbySybengaand plainedexplicitlybythescenarioproposedabove,ifonefur-
colleagues(e.g.,Sybengaetal.1994),althoughnotelaborated ther assumes a strong tendency for first CO designations to
in a context of recent advances in understanding of this occur near chromosome ends (e.g., as in barley diploid
process. meiosis;above).
Otherpossibilities?
Supportingevidence
Whatother modulationsof the recombination process might
Severaladditionalobservationssupporttheabovescenario. allow evolution of stably fertile autotetraploid species? It
MI bivalent associations seen in well-evolved autotetra- mightbeimaginedthatthenumberofCOsmightbereduced
ploidsofteninvolveoneandonlyonechiasma,versuslarger in other ways. One possibility would be a reduction in the
numbersofchiasmataperbivalentintheirdiploidprecursors number of total recombination interactions. However, this is
(e.g., above; Dawson 1941; Mulligan 1967; Gillies 1969; unlikely to be a major factor because ofthe phenomenon of