Expression of the Engrailed Gene Reveals Nine Putative Segment-Anlagen in the Embryonic Pleon of the Freshwater Crayfish Cherax destructor (Crustacea, Malacostraca, Decapoda)

9 Pages·1995·3.6 MB·English

by  G Scholtz

Reference: fi/W. Bull 188: 157-165. (April. 1995)
Expression of the Engrailed Gene Reveals Nine
Putative Segment-Anlagen in the Embryonic Pleon
of the Freshwater Crayfish Cherax destructor
(Crustacea, Malacostraca, Decapoda)
GERHARD SCHOLTZ*
School ofBiological Science, The University ofNew Soul/i Wales.
P.O. Box I, Kensington, NSW'2033, Australia
Abstract. Segment formation in the embryonic pleon characteristic tagmosis. the position ofthegonopores, the
ofthe freshwatercrayfish Cheraxdestructorwasanalyzed subdivision ofthe stomach into specific functional units,
by usingthe monoclonal antibody mAb 4D9 against the and a ringof 19 embryonic ectoteloblasts (forreview and
productofthesegment-polaritygeneengrailed. Asinother discussion of different views see Dahl. 1992; Wagele,
body regions, engrailed is expressed in transverse stripes 1992). Within the Malacostraca. the Leptostraca possess
in the posterior portion of segments in the pleon. Nine a pleon (abdomen) consisting of seven segments and a
engrailedstripesare formed in thepleon. Theanteriorsix telson. In contrast to all anterior segments, the seventh
stripes correspond to the six pleon segments ofadult eu- pleonic segment is limbless. The other malacostracan
malacostracan crustaceans. The uropods are clearly the groups, unified as the Eumalacostraca (sensu Grobben,
appendages ofthe sixth pleon segment. The seventh en- 1892). have only six pleomeres. all equipped with limbs.
grailed stripe marks the anlage of a seventh ganglion. The posteriormost limbs arethe uropods which, together
Stripes eight and nine are transient and disappear before with the flattened telson, form the tail fan. The general
morphogenesis begins. The engrailedstripesseven to nine view is that the pleon of the Leptostraca represents the
are interpreted as vestigesofancestral segments. Thesev- plesiomorphic condition, and the lossofa pleon segment
enth segment anlage isthusa recapitulation oftheseventh and the evolution ofthe tail fan are considered to be de-
pleonic segment, which is retained in recent adult leptos- rived characters ofthe Eumalacostraca (e.g., Lauterbach,
tracans and is considered to be part ofthe malacostracan 1975; Hessler, 1983). However, there hasbeen somecon-
ground plan. The stripes eight and nine might point still troversy over which pleomere has been lost in the course
further back into the phylogeny of crustaceans or even ofevolution and to which segment the uropods belong.
mandibulates. The use of rhodamine-labeled phalloidin On the basis of anatomical and paleontological data.
reveals that the terminal ganglion ofadult crayfish is the Siewing (1956. 1963) argued that the uropods might be
fusion product of the anlagen of the sixth and seventh theappendagesoftheseventh pleomereandthatthesixth
pleonicganglia andan eighth hemiganglion thatisdevoid (penultimate) pleomere has been lost in most eumala-
ofengrailedexpression. costracans. Based on her embryological studies in mysi-
daceans. Manton (1928a. b). in contrast, suggested that
Introduction the uropods belong to the sixth pleomere and that the
The Malacostraca constitutes a monophyletic taxon original seventh is fused to the sixth pleomere. Although
well defined by several apomorphic characters such as a it was shown in the meantime that paleontological data
do not support Siewing's suggestions (Dahl. 1983) and
although Manton's view has been adopted by many car-
*RePcreeisveendt 1a5ddNroesvse:mbInesrtit1u9t94f;uracZcoeopltoegdie1.8JFareniuearyUni1v9e9r5s.itat Berlin. cinologists such as Lauterbach (1975), Hessler(1983). and
Konigin-Luise-Str. 1-3, D-14195 Berlin, Germany. Dahl ( 1992) the problem is still far from being settled
157
158 G. SCHOLTZ
(see, for instance, the discussion after the 1983 paper by and fifteenth rows(eXIVandeXV). Eachrow(eltoeXIV)
Hessler) and a different approach seemed to be required cleavestwice, formingfourregularlyarrangeddescendant
to clarify this issue. rows. The intersegmental furrow, separatingtwoadjacent
Within the arthropods, insects and crustaceans have segments, isformed within thedescendantsofoneoriginal
the segment-polarity gene engrailedalready expressed in ectoderm row. In contrast to all other rows, row fifteen
transverse stripes at the posterior margin of embryonic cleaves rapidly several times, forming a field ofcells in a
segments before morphogenesis takes place (e.g.. Patel et grid-like arrangement.
al. 1989; Manzanares ct at.. 1993; Scholtz et al. 1993, In Drosoplula. the segment-polarity gene engrailed
1994; Patel, 1994). Therefore, it is a suitable marker for plays a crucial role in specifying the fate of the cells in
the analysis ofthe terminal regions ofarthropods where the posterior part (compartment) ofsegments and in es-
segmentation is obscured by morphological rearrange- tablishing the segmental boundaries (Lawrence, 1992).
ments and the loss ofsegmental structures in the adult. The expression pattern ofengrailedK very similar in dif-
Several studies have used engrailed to analyze the seg- ferent insect and crustacean species. Therefore, a con-
mentation of the head in insects and crustaceans (e.g.. served function ofthe engrailedgene throughout the ar-
Fleig, 1994; Schmidt-Ott et al.. 1994; Scholtz, 1995), but thropods has been suggested (Patel. 1994). The basic
the caudal segments have been analyzed only in insects modes ofpleonic engrailedstripe formation described in
(e.g., Kuhn et al., 1992; Schmidt-Ott et al.. 1994). the followingcorrespond tothose reported forotherbody
In the present investigation. I used the anii-engrailed regions of Cherax (Scholtz et al.. 1993; Scholtz. 1995)
antibody mAb4D9 (Patel et al., 1989) toanalyzethe seg- and forothercrustacean species(Patel et al.. 1989; Scholtz
mentation ofthe embryonic pleon ofa eumalacostracan, et al.. 1993. 1994). In the post-naupliar germ band of
the Australian freshwater crayfish Cherax destructor. I Cherax and other malacostracans, engrailedis expressed
foundthat, posteriortothesix pleomerestypical foradult in the anterior descendants of each ectoderm row, and
eumalacostracans, three additional vestigial segment an- the intersegmental furrow is formed posterior to the en-
lagen occur in front of the telson. Furthermore, a true grailedstripes.
seventh pleonic ganglion and an eighth partial ganglion
are formed embryologically. These fuse with the sixth Theformation <>t engrailed stripes
pleonic ganglion anlage to form the terminal ganglion of
the adult animal. I interpret these findings as recapitula- Nine engrailed stripes are formed in the embryonic
tions ofancestral characters, and they shed new light on pleon of C. destructor (Fig. 1). The first pleonic stripe
the evolutionary transformation of crustacean segmen- appears in ectoderm row elX and the sixth stripe in row
tation patterns. eXIV. Stripes seven to nine are formed within the deriv-
atives ofrow eXV.
Material and Methods Thestripesappear in astrictly anteroposteriorsequence
(Fig. 1). At about 40% to 42% development the first en-
The rearing and maintenance ofembryos ofthe Aus- grai/edstripe is formed and indicates the posterior margin
tralian freshwater crayfish Chera\ destructor were de- of the prospective first pleomere (Fig. 1A). The ninth
scribed by Sandeman and Sandeman (1991). Theirpaper pleonic engrailed stripe appears at about 65%> develop-
amlesnotdeafnidnesthtehepoesmtbermybornyiocnisctasgteasgeisn(pee.gr.c,enPtOofI)dtehvaetloapr-e smtearntts(cFliogs.e ItoD,thEe).mTidhleinfeoramnadtipornocoefeedasclhatienrdailvlyid(uFailg.st1rDi)p.e
used in the present investigation. Immunocytochemistry The initial distance between the last two engrailedstripes
and fluorescent stainingweredescribed in detail in Scholtz ofany stage is one row ofengrailednegative cells. This is
et al. (1994) and Scholtz (1995). also true for stripes seven to nine (Fig. IE). Stripes one
to seven are associated with the complex metamerically
Results repeated cleavage pattern in the post-naupliargerm bands
A short summary oj'previous investigations ofCherax. The cell division pattern in the area ofstripes
eight and nine, although not analyzed in detail, is some-
The cell lineage in the ectoderm ofthe germ band of what different (Scholtz, 1992).
Cheraxwasdescribed in apreviousstudy(Scholtz, 1992). Initially, all stripes are one cell wide (Fig. 1 ). Also, at
As in most other malacostracans, the largest part ofthe least pleonicstripesonetoeight(notconfirmed forpleonic
germ band isformed by stem cells in the posteriorgrowth stripe nine) pass through a transient widening phase
zone,theectoteloblasts. Theectoteloblastsproducetrans- caused by divisions of the engrailed-posilive cells (Fig.
verse cell rows in an anteriordirection by highly unequal IB, C). The widening phase is followed by narrowing to
divisions. Thirteen ofthese rows are formed (el to eXIV) a one-cell width again due to the lossofengrailedexpres-
before theectoteloblastsdivide equally into the fourteenth sion in posteriorcells in the stripe (Fig. 1B, C). After nar-
EMBRYONIC PLEON OF CRA^i ISM 159
I
V
te
Figure 1. Formationofengrailedstripes:st.stomodeum;cp.caudal papilla;te. telsonanlage: up. uropods.
(A) Germ band at 40% to 42% development. All engrailed stripes of head and thorax are formed. The
engrailedstripe markingthefirstpleonicsegmentappears(arrow)ontheventralsideoftheventrally Hexed
caudal papilla. (B) Posterior part ofthe caudal papilla ofan embryo at 45% development (ventral view).
The engrailedstripes up to the third pleonic segment (1, 2. 3) are formed. The posteriormost stripe (3) is
onecell wide. The next anteriorstripe(2)isin the narrowingphaseafterthe initial widening. Thestripeof
the first pleonicsegment (1) is in the phase ofsecondary widening in correlation with the formation ofthe
intersegmentalfurrow andthelimbbuds(fordetailsseeScholtzcm/.. 1993). (C)Posteriorpartofthecaudal
papillaofanembryoatabout60% development(ventralview);engrailedstripessevenandeightareformed.
Stripeeight isin thefirstwideningphase;stripeseven hasalready narrowed toonecell width. The uropods
begin to appear in the area ofthe sixth engrailedstripe (sixth pleonic segment). Nole the buds ofthe first
pleopods(arrow),whicharelackingintheadultsofCherax(comp. Fig. ID).(D)Posteriorpartofthecaudal
papillaofanembryoatabout65%development(ventralview). Theninthengrailedstripeappearsfollowing
a mediolateral sequence ofengrailed expression. The ninth stripe is the posteriormost area ofengrailed
expression, the posterior margin ofthe telson anlage does not express engrailed. The arrow points to the
buds ofthe first pleopods. (E)Closeup ofthe same preparation as in (D); 7 to 9 seventh to ninth pleonic
engrailedstripes. (F) Posteriorpartofthecaudal papilla ofan embryoat 70'v development(ventral view).
Neuronal engrailedexpression begins in the area ofthe seventh pleonic stripe (7). The cells ofengrailed
stripeeight begin tocease engrailedexpression; stripe nine hasdisappeared.
160 G. SCHOLTZ
rowing, stripes one to seven widen during the morpho- (Fig. 1). This isalso true forpleonic stripesone tosix, but
genesis of segmental structures such as intersegmental stripes seven to nine are not related to the formation of
furrows, ganglia, and limb buds(Fig. 1). Stripeseight and intersegmental furrows (Figs. ID, E. 2). In segments one
nine disappear before widening takes place (Fig. IF). tosix, limb budsare formed whose posterior portionsare
Stripesonetosix surround thecaudal papilla and form also engrailed positive (Figs. 1, 2). No limb buds occur
complete circles ofabout 40 etwui/ecl-positi\e cells (Fig. in segments seven to nine, and in the corresponding re-
1, 2A). Thus, engrailed is expressed in the midventral gions the cells do not show engrailedexpression (Figs. 1,
neurogenic region, in the lateral limb budding area, and 2). Interestingly enough, embryonic limb budsare formed
in the dorsal portion ofthe forming segments one to six. in the first pleonic segment (Fig. 1C, D) where the adult
Stripes seven to nine are restricted to the medioventral ('. iteMnielor. like all parastacid crayfish species, isdevoid
part, which includes the neurogenic region (Fig. ID, E). of appendages in both sexes (Hobbs, 1988). The limbs
They consist ofseven to nine engrailedexpressing cells. associated withthesixthpleonicengrailedstripeareclearly
Stripesonetosevenshowatwofoldengrailedexpression the posteriormost appendages on the germ band (Figs.
in the embryonic epidermis and in the forming ganglia 1C, D, E. 2). On the basis of their shape in advanced
(Figs. 1, 2). In the stages examined, the superficial epi- developmental stages,theycan beidentifiedtobetheuro-
dermal engrailed expression continues in the forming pods (Figs. 1.2).
limbs and the dorsal portions ofthe segments (Fig. 2). In
the neurogenic area ofadvanced stages, engrailedexpres- Neurogenesis
sion is restricted to individual neuronal precursors and
neurons, whereas the superficial engrailedexpression has In the pleonic segments one to seven, engrailed is ex-
disappeared (Figs. 1, 2). In contrast to this, stripes eight pressed in cellsofthe formingganglia(Figs. 1,2). Thereby,
and nine appear only transiently in the embryonic epi- the arrangement ofengrailed-positive cells (neurons ?) is
dermis (Figs. 1, 2). very similar between the seventh and more anterior seg-
ments (Fig. 2). The engrailed expression in stripes eight
Morphogenesis and nine disappears at about 70% to 75% development
In all other body regions, the intersegmental furrows and is not correlated with neurogenesis. The staining of
are formed immediately posterior to the engrailedstripes the embryonic central nervous system with rhodamine-
Figure 2. tarly neuronalengrailedexpression inthepleon: up. uropods.(A)Posteriorpartofthecaudal
papilla of an embryo at about 75% development (ventral view; brightfield micrograph). The pattern of
I'li^ruiU'Jexpressioninneuronalprecursorsandearly neuronsin theseventh pleonicganglionanlageisvery
similar to lhat ofmore anterior pleonic ganglion anlagen, indicating a serial homology between all seven
pleonicganglia. Theresemblanceconcerns, in particular,amedianengrailed-positivecell(arrowheads)and
asetofthree inlenscK stainedcellsat theposteriormargin oftheengrailed-positivearea(compareFig. 2C).
Notethedorsalengrailedexpression inpleomeresonetosix.(B)Samepreparationasin(A),withNomarski
optics.Thistechniqueshowsthebudsofthepleoniclimbs.Theuropodsarebeginningtogettheircharacteristic
shape and are clearly connected with the sixth pleomere. (C) Closeup ofanother preparation (Nomarski
optics). The serially homologous group ofthree intensely stained cells in ganglion anlage seven and more
anteriorganglion anlagen is marked byarrowheads.
FMBRYONIC PI EON OF CRAYFISH 161
labeled phalloidin reveals that a true seventh embryonic malacostracan taxa including Leptostraca (Claus, 1888;
ganglion is formed in addition to the anterior six pleonic Manton, 1928b, 1934), Hoplocarida (Shiino, 1942), Syn-
ganglion anlagen (Fig. 3). All pleonic ganglion anlagen carida (Hickman, 1937), Mysidacea (Manton, I928a, b),
one to seven share the occurrence oftwo main commis- Tanaidacea (Scholl, 1963), and Isopoda (Stromberg,
sures and a characteristic median Y-shaped neuron that 1967). No seventh pleonic ganglion occurs in theembryo
might correspond to the neuron S described by Whiting- ofthe amphipod (.iammarus pu/e.\", however, for a short
ton el at. (1993) (Fig. 3A. B). Posteriorto the seventh, an time during development, the anlage ofthe sixth pleonic
eighth ganglion anlage is formed. It possesses only one ganglion is subdivided into two distinct adjacent areas
commissure, and the characteristic median cell is lacking a phenomenon interpreted as a vestigial formation ofa
(Fig. 3B). Furthermore, no neuronal engrailedexpression seventhpleonicganglion (Weygoldt. 1958). Noneofthese
occurs. From this eighth hemiganglion, two nerves run authors hasmentioned thepartial eighth pleonicganglion,
posteriorlytowardstheembryonictelson region (Fig. 3B). but this might be due to the techniques used (no whole-
During further ontogenesis the anlagen of ganglia six, mounts or horizontal sections). Adult eumalacostracans
seven, and eight fuse and become a morphological unit possess only six pleonic ganglia (see Hanstrom, 1928),
that formsthe terminal ganglion ofthe adult animals(Fig. and this is also true forleptostracans with aseventh pleo-
3C). Thereby thecommissures remain separated. Theco- mere (Claus, 1888; Manton, 1928a). Against this back-
alescence ofthese ganglion anlagen isclearly visible from ground, I conclude that the composite nature ofthe ter-
about 80% development. However, engrailed expression minal (sixth)ganglion and the pattern ofitsformation by
still indicatesthecomposed origin oftheterminal ganglion fusion duringembryogenesis is part ofthe malacostracan
in postembryonic stages (e.g., PO I) (Fig. 4). ground plan. Thus, the fusion ofthe terminal ganglion is
apparently not correlated with the evolution ofthe uro-
Discussion pods and their complex function in eumalacostracans.
Development ojthe terminalganglion
Origination of the uropodsfrom the sixth pleomere
The present investigation shows that the terminal gan-
glion ofthe adult parastacoid crayfish Cherax destructor The anterior six engrailed stripes in the embryonic
is the fusion product ofthree embryonic ganglion anla- pleon ofCheraxmark theposteriorborderofthesix pleo-
gen the sixth and seventh pleonic ganglia and a partial meres that persist in the adult. Engrailedis expressed in
eighth ganglion. Thisdevelopmental pattern corresponds theventral region comprisingtheganglion primordia and
to that described for the astacoid crayfish Procambarus the limb buds as well as the lateral and dorsal sides of
clarkii (Dumont and Wine, 1987). Furthermore, these each segment. The seventh to ninth pleonic engrailed
embryological data are consistent with results from neu- stripes are restricted to the ventral side of the embryo.
roanatomical and immunohistochemical studies analyz- However, from the mode offormation and the distance
ingtheterminalganglia oftheadultsofseveral freshwater between them, they correspond to the anterior stripes.
crayfish species (Stoll, 1925; Kondoh and Hisada, 1986; Thus I conclude that all pleonic engrailedstripesare serial
Audehn et at., 1993) and ofthe lobster Homams gam- homologues and that stripes seven to nine also indicate
mams (Winlow and Laverack, 1972). The embryonic segment anlagen.
morphology and the pattern of engrailed expression of The pattern ofthe sixth engrailedstripe clearly reveals
the seventh pleonic ganglion resemble to a high degree that the uropodsofClieraxarethelimbsofthesixth pleo-
those ofthe anterior pleonic ganglia. This suggests that it mere, which is also true for the mysid Neomysis integer
represents a true segmental ganglion homologous with (unpub. obs.). This resultconfirmssomeofthesuggestions
the anterior segmental ganglia. The eighth pleonic gan- of Manton (1928a, b), and since there is good evidence
glion anlage shows only one commissure. Furthermore, that the tail fans ofall eumalacostracan groups are ho-
this ganglion lacks any engrailedexpression which char- mologous(Hessler, 1983; Wa'gele, 1994).thefindingspre-
acterizes the posterior part offorming ganglia, although sented here might also be valid for eumalacostracans in
an epidermal eighth pleonic engrailed stripe is formed. general. Therefore. Siewing's (1956, 1963) hypothesis that
Taken together, this suggests that the eighth pleonic gan- the uropods originate from the seventh pleon segment
glion ofthe embryo might be the anterior part ofa true cannotbe maintained. The present resultsalsocontradict
segmental ganglion anlage. On thebasisoftheoccurrence the assumption that the caudal rami ofthe telson oflep-
ofspecifically arranged identified neurons. Audehn et al. tostracans are homologous with the eumalacostracan
(1993) came to similar conclusions. uropods (Bowman, 1971; for further arguments against
The formation of an embryonic anlage of a seventh this view see Schminke, 1976). Furthermore, the telson
pleonic ganglion that later fuses with the sixth ganglion ofdecapods does not correspond to the ancestral seventh
has been reported from representatives of most higher pleomere, aswassuggested by Kondoh and Hisada(1986).
162 G. SCHOLTZ
Figure3. Gangliogenesisasseenwith rhodamine-labeled phalloidin: st.stomatodeum;pr.proctodeum;
pg2, second pleonic ganglion; pg5. fifth pleonic ganglion; tg. terminal ganglion. (A) The nerve cord ofan
embryo at 7y, to SO', development. The black arrow points to the eighth thoracic ganglion anlage; the
whitearrow indicates theeighth pleonic(hemi)ganglion anlage. (B)Closeup ofposteriorganglion anlagen.
The seventh ganglion anlage(asterisk)showsthesame pattern asthoseofmoreanteriorsegments(t'.#. the
sixthganglionanlage;star).Theeighthganglionanlage(largewhitearrow)consistsofonlyonecommissure.
The black arrow showstheeighth thoracicganglion anlage;the small whitearrowspoint to the median Y-
shapedneurons.(OAdvancedstage(85%to90%development)showingthatthesixth(star),seventh(asterisk),
andeighth(whitearrow)ganglionanlagenareembeddedinamorphologicalunit,formingonelargeterminal
ganglion.
The additional engrailed stripes seven to nine lie clearly The seventh pleonic engrailed stripe in the embryo
in front ofthetelson anlage, which ischaracterized by the otCherax is an obvious example ofthe recapitulation
proctodeum andwhich lacksengrailedexpression. Against ofancestral conditions(seeSudhausand Rehfeld, 1992).
thisbackground, theseemingly"seven-segmented" pleon It demarcates the posterior border of an additional
with uropods originating from the "seventh" pleomere of (seventh) pleonic segment that is missing in adult cray-
some lophogastrids is not plesiomorphic, as considered fish and other eumalacostracans but is present in adult
by Manton (1928b), Siewing (1956). and Lauterbach Leptostraca, the sister-group of the Eumalacostraca.
( 1975), but is a derived feature. There is ontogenetic and There is no reason to assume that pedomorphosis led
phylogenetic evidence for this suggestion. Manton's to the occurrence ofthe seventh segment in adult lep-
(1928b) hypothesis that the uropods migrate posteriorly tostracans and to the loss of the highly complex eu-
beforetheborderbetween thelasttwosegmentsisformed malacostracan tail-fan (Messier. 1983; Fault'/ al.. 1985)
is not consistent with the finding that the intersegmental in this group.
furrows are formed before limb buds appear (Scholtz, Interestingly, in the crayfish as in other eumalacos-
1990; present investigation). We know from Drosophila tracans, the corresponding seventh pleonic ganglion
genetics thatthe establishment ofthe segmental borderis persists and is fused with the sixth to form a morpho-
the prerequisite for the subsequent differentiation ofseg- logical and functional unit (see above). In addition,
ments(Lawrence, 1992). Therefore, the "seventh pleonic some authors report that the embryos of several ma-
segment" of lophogastrids is apparently the result of a lacostracan species contain terminal mesodermal so-
secondary nonsegmental(?) subdivision of the terminal mitesthat might be related to a vestigial seventh pleonic
eumalacostracan segment, as was suggested earlier by segment and thatalso fusewiththesixth pleonicsomites
Claus(1888). With respect tothe position oflophogastrids (e.g.. Manton, 1928a; Shiino. 1942). Those processes
in the eumalacostracan phylogenetic tree (e.g.. Siewing, can be characterized as fusions, but fusion does not
1956; Richter, 1993), it is more likely that a subdivision seem to be the appropriate description forevents in the
ofthe terminal segment occurred only once in the lopho- superficial segmental parts. The pleonic engrailedstripe
gastrid line than that the seventh pleomere has been lost seven (like stripeseight and nine) is more likea transient
independently in several eumalacostracan lines. segment anlage that is not involved in morphogenesis
EMBRYONIC PLEON OF CRAYFISH 163
Figure4. Expressionofengrailedinthepleonicnervecordofthefirstpostembryonicstage(POI): pgl.
first pleonic ganglion: pg4, fourth pleonic ganglion; pg5. fifth pleonic ganglion; tg. terminal ganglion. (A)
Thesixgangliaofthepleonofthefirststageafterhatching(POI)(ventralaspect). (B) Highermagnification
ofthe same preparation, showing the ganglia ofthe fourth and fifth pleomeres. They exhibit a serially
repeated pattern oftwo areas ofengrailedexpression. An anterior region ofcells (neurons, glia, or their
precursors) with relatively small nuclei (asterisks)and a posterior region with large engrailed-positive cells
(arrows).(C)Theterminalganglion(samepreparationasin(A)).Thepatternofengrailedexpressionreveals
thecomposed natureofthisganglion. Theasterisks indicate theanteriorsegmental engrailedareaand the
arrowspointtotheposteriorsegmentalengrailedarea(compareFig.4B).Thepatternofengrailedexpression
intheseventhganglion anlage(7)issomewhat reduced when compared with that ofthesixth ganglion an-
lage(6).
and that disappears during further development. From homonomously segmented trunk (Hessler and Newman,
the outset, the morphological border between the ter- 1975; Schram, 1982; Walossek, 1993) to animals with a
minal segment and the telson lies behind the sixth distinct subdivision ofthetrunk intothoraxandalimbless
pleonic engrailed stripe. abdomen (Lauterbach, 1986; Fryer, 1992). But until the
phylogenetic relationships between the highercrustacean
Phylogemtic significance ofpleonic engrailed stripes taxa are resolved see Siewing (1963), Schram (1986),
eight and nine and Wilson (1992) for various proposals the recon-
The eighth and ninth pleonic engrailedstripes are also struction of a crustacean ground plan (sensit Hennig.
1966) will be pure speculation.
considered toindicate vestigial segmentsthatrecapitulate
ancestral conditions. Thissuggestion is based on the sim- Nevertheless, thepresent findingspermitsometentative
ilar appearance of stripes seven to nine and on the fact conclusions. The occurrence ofadditional segment rem-
that many non-malacostracan crustaceans possess more nants in the embryonic pleon of Cherax argues against
segments than malacostracans. However, it is difficult to an original number of 15 trunk segments in crustaceans
sayhowfarbackstripeseightand ninepointin phylogeny or even mandibulates as suggested by Walossek (1993);
and in which ancestral lineage these segments have been the numberoftrunksegmentsin thecrustacean stem spe-
lost in the adults. The question ofwhether segmentation cies must have been higher. Therefore, the additional en-
and tagmatization of the Malacostraca are primitive or gniiledstripesin the pleon ofChera.\ratherargue in favor
are derived within the Crustacea has been debated, and of Lauterbach's (1975) hypothesis ofa loss ofposterior
the many attempts to reconstruct the crustacean stem segments in the ancestral lineage ofmalacostracans. The
species have yielded verydifferent resultsconcerningtag- restriction ofthese additional stripes to the neural region
mosis and segment number. The proposals reach from and the entire lack oflimb anlagen furthermore support
short animals with only a few segments (Miiller, 1864)to the suggestion that these segments are vestiges of the
forms with many segments (Hesslerand Newman. 1975: limbless abdomen postulated by Lauterbach (1986) and
Lauterbach, 1986), and from forms with a more or less Fryer (1992) for the crustacean stem species.
164 G. SCHOLTZ
Acknowledgments Manlon, S. M. 1928a. On the embryology of a mysid crustacean,
Hemimysislamornae. Phil Tram. R Soe Loiul IB) 216: 363-463.
I thank David Sandeman and Renate Sandeman for Xhiiitdii. S. M. 1928b. On some points in the anatomy and habitsof
their helpful advice as well as for the opportunity to use thelophogastridCrustacea. Tram. R Soe. Etlin 56: 103-1 19,3Plates.
theircrayfish cultureand towork in theirlaboratory. The Manton. S. M. 1934. On the embryology ofthe Crustacean \ehuliii
aPnatteil-.engIrauimli'gdraatneftuilbotdoySwtaefsana gReicnhetreoru,sWgoifltffgraonmg NDiohplaem, Man/zhiaaptnei-ao,rnePashn,idlMd.eT,ivie/Ril.fo,pMmaRerncStoao,leapnIa.dt,tmRei.rlnG1oa11fr)ees2xs2pe3r:.ess11i96o93n3-.2o3f8t.Gheeneioimxiniciloerig/agnein-e
and David Sandeman for critically reading the manu- from the brineshrimpArlenini Development 118: 1204-1219.
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