DNA Amounts in Two Samples of Angiosperm Weeds

14 Pages·2003·1.23 MB·English

Annals of Botany 82 (Supplement A): 121-134, 1998
Article No. bo980785
DNA Amounts in Two Samples of Angiosperm Weeds
MICHAEL D. BENNETT*, ILIA J. LEITCH and LYNDA HANSON
Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, UK
Received: 3 August 1998 Returned for revision: 15 September 1998 Accepted: 21 September 1998
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Of the world's 250000 angiosperm species, only about 200 are recognized as important weeds. 4C nuclear DNA lo
amounts were estimated for 39 such species. Success for many important weeds is suggested to reflect several traits ad
known to correlate with low DNA C-value, so such weeds may have smaller DNA C-values than other species. Our ed
work tests this hypothesis, comparing DNA amounts in 156 species recognized as important world weeds or British fro
garden weeds, with 2685 other species. DNA amounts did not differ significantly between the two weed samples, but m
weeds showed highly significant differences from other species. For example, nuclear DNA amount in weeds (mean h
11 74 pg) was smaller than in other species (mean 28 13 pg), and restricted to the lowest 20 % of their range. Similarly, ttp
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DNA amount per genome in weeds (mean 3-79 pg) was smaller than in other species (mean 1214 pg), and restricted ://a
to the lowest 10 % of their range. As significant differences between weeds and other species remain for almost all sub- c
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samples tested, this contrast is widely distributed. So it is important to ask how selection against high nuclear DNA d
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amount and genome size in weeds operates. The probability of a species being a weed fell significantly with increasing m
nuclear 4C DNA amount, and mean genome size, reaching zero just above 100 pg, and 19 pg, respectively. Moreover, ic
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polyploidy was significantly more frequent in weeds (51 %) than in other species (27 %),i ncreasing with nuclear DNA u
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amount in both, reaching 100% in weeds with the highest 4C DNA amounts, but only 41 % in other species. Thus, .c
selection for polyploidy in weeds may partly reflect their increased genetic variability, independent of DNA amount. om
However, such selection pressure grows strongly with rising nuclear DNA amount, and this may act mainly on /a
correlated factors including faster development. © 1998 Annals of Botany Company ob
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Key words: Angiosperm DNA amounts, DNA C-values, genome size, important world weeds, British garden weeds, rtic
polyploidy. le-a
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fill this gap, and to facilitate meaningful comparisons of t/8
INTRODUCTION DNA C-values in important weeds and other angiosperms. 2/su
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A weed can be defined as 'a plant or a group of plants which The success of different weed species is suggested to reflect p
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is not desired at its place of occurrence' (Pieterse and many different traits (Baker, 1974). These include the ability 1
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Murphy, 1990). Of the world's 250000 known flowering to: (1) establish quickly, and display a short juvenile phase; 2
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plant species, only about 200 are recognized as important (2) develop rapidly throughout the life cycle, and have a /2
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weeds (Holm et al., 1997), yet they have a vast impact on a short minimum generation time; and (3) produce rapidly 11
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global scale. The most important weeds are those which many small seeds in many highly invasive species (Oka and 9
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interfere with food production. It has been estimated that Morishima, 1982; Rejmanek, 1996). All of these characters y
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weeds cause 90 % of the losses of food in agriculture around are known to correlate with low DNA C-value, especially in u
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the world (Holm et al., loc. cit.). time-limited environments (Bennett, 1971, 1972, 1976, 1987; st o
While much has been written on the identity, biology and Bennett, Smith and Lewis Smith, 1982). Thus, taking these n
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distribution of weeds (Holm et al., 1977, 1979, 1997), a various nucleotypic relationships together, it seems reason- 6
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recent survey of nuclear DNA C-values in angiosperms able to predict that, as a class, successful weeds may tend to o
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noted an important gap in our knowledge. Since the 1950s, have smaller DNA C-values and a lower range of genome e
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much work has been done on nuclear DNA amounts in a sizes than other species. The present work provides a first b
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wide variety of plants. Such data are available for just over test of this hypothesis. r 2
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1% of known angiosperm species (Bennett and Leitch, 1
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1997). Interest in this highly variable biodiversity character
has remained high, with C-value data being used in many MATERIALS AND METHODS
fields of biological research. However, there are still many
Materials
gaps in the database, and representation of many taxonomic
groups, geographical regions and plant life forms is poor While the ideal test might seem to be between weed and
(Bennett and Leitch, 1995). In particular, it was noted that non-weed species, achieving this is not straightforward,
nuclear DNA amounts for many important weed species because the definition of a weed ('a plant... which is not
were unknown. The present study was undertaken to help desired at its place of occurrence') is so vague as to allow
almost any species to be classified as a weed. To facilitate
meaningful tests it was decided: (1) to compare DNA C-
* For correspondence. Fax [phone], e-mail m.bennett@
rbgkew.org.uk values for only species clearly recognized as weeds by
0305-7364/98/0A0121 + 14 $30.00/0 © 1998 Annals of Botany Company
122 Bennett et al.--DNA Amounts in Angiosperm Weeds
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126 Bennett et al.-DNA Amounts in Angiosperm Weeds
leaders in the field of weed biology, with those for other come from 36 families. The latter include 21 families
species; and (2) to restrict such a comparison to two samples represented by only one or two species, but eight or more
which differ in being recognized as of major world, or of species from only five families including Ranunculaceae (11
particular local, significance. Thus, it was decided to species) and, perhaps as expected, several large families such
compare species listed as either (1) 'important', 'problem', as Gramineae (45 species), Compositae (18 species), and
or 'the world's worst weeds' in one or more of five key texts Leguminosae (eight species). Given their variability and
(namely: Holm et al., 1977, 1979; Pieterse and Murphy, size, the above samples seem sufficiently representative of
1990; Caseley, Cussans and Atkin, 1991; Harr et al., 1991), such angiosperm weeds as to allow meaningful comparisons
or (2) British garden weeds in one or more of four other key and conclusions.
texts (Morse and Palmer, 1925; Chancellor 1980, 1994;
Ailes, 1981), with (3) those for all other species. These
Estimating total nuclear 4C DNA amounts D
samples are referred to below as important world weeds o
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(IWW), or British garden weeds (BGW). It is realized that Root tips of the 44 test species and calibration standards n
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some 'other species' may sometimes be weeds causing were fixed simultaneously in 3:1 ethanol :acetic acid, and ad
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smaller or local problems. However, their presence in the stored at -20 °C for up to 3 weeks. Various calibration d
large sample of 'other species', where the vast majority are standard species were used, each chosen mainly to have a C- fro
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non-weeds, does not prevent meaningful comparisons. value similar to that of the unknown species being measured. h
Together, appropriate key texts (Holm et al., 1977, 1979; Roots of the test and standard species were rinsed briefly in ttp
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Pieterse and Murphy, 1990; Caseley et al., 1991; Harr et al., distilled H20, then hydrolysed for 40 min in vials containing ://a
1991) list about 200 of the world's worst weed species. a volumetric solution of 5 M HCI (Merck, product no. ca
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Nuclear DNA estimates for 53 of these were already 190667T) in a water bath at 25 C. After hydrolysis, roots e
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available (Table 1). Seeds of a further 44 such species (Table were rinsed briefly in distilled HO, then stained in the dark ic
1) were obtained from either the Seed Bank at the Royal for 2 h in pararosaniline (Sigma, UK) at 23 C. The roots .ou
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Botanic Gardens, Kew (Wakehurst Place, UK), or Herbi- were then washed for 3 x 10 min in SO2 water and finally .co
seed Nursery (Wokingham, UK). These were germinated in transferred into vials containing distilled water, and kept m
incubators on 1% agar in plastic Petri dishes (90 mm overnight at 4 C. Slides were made next morning, and /ao
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diameter) under a wide range of different species-specific estimates of 4C-values in ten mitotic nuclei at prophase on /a
nutrient, growth factor, temperature and day length each of three slides were made using a Vickers M85a rtic
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conditions, too varied to describe in detail here. Thus, the microdensitometer. 4C-values of the calibration standards -a
present sample of IWW comprised a total of 97 species. used to convert the arbitrary units into absolute amounts bs
Although not listed as IWW in the above key texts, other were taken from Bennett and Leitch (1995). Where possible, tra
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species are recognized as weeds in other contexts, e.g. locally chromosome numbers were also counted from mitotic t/8
2
in horticulture. The four key texts (see above) list 135 British metaphase spreads on these slides (i.e. for 29 species-see /s
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garden weeds, to which Arabidopsis thaliana (L.) Heynh can Table 1). p
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be safely added. Nuclear DNA estimates for 74 of these Seedlings of some species were potted up and grown on in l_1
were already available (Table 1). Comparison showed that a glasshouse for taxonomic verification. Of the 44 species /12
1
20 of the above sample of 97 IWW were also listed as BGW, whose DNA C-values were estimated in the present work, /2
1
including five whose DNA amounts are estimated in the 18 were measured from seed whose taxonomy was previously 1
1
present work (Table 1). Thus, the sample of BGW of known verified (List of seeds 1996, Seed Bank, Wakehurst Place, 1
9
C-values totalled 79 species, while the total number of compiled by S. Linington). Vouchers of a further 18 species by
species listed as IWW, BGW, or both, was 156 (Table 1). were prepared and placed in the Royal Botanic Gardens gu
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These samples comprise considerable variation (Table 1), Kew, Herbarium (K). Vouchers of the remaining eight s
e.g. the 156 weeds contain 116 species of known ploidy, species will be deposited when the plants have flowered. t on
including 57 diploids (49 %) and 59 polyploids (51 %), and 16
IWW and BGW separately contained similar proportions of N
diploids, and ranges of polyploids. Among IWW, 33 out of Genome size ove
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68 species of known ploidy level (48.5 %) were diploids, as The present study compares DNA amounts for post- b
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were 30 out of 65 species (46.2 %) among BGW. While 39 replication nuclei or genomes. The former is correctly r 2
tetraploids (336%) and six hexaploids (52%) accounted known as total 4C DNA amount per nucleus, but it can be 01
8
for most polyploidy overall, two species at higher ploidy incorrect or impossible to refer to genome size in terms of C-
levels occurred in both IWW and BGW, with a maximum of value. Whereas C-value equals the unreplicated genome size
one octoploid among IWW, and one 16-ploid among BGW. in diploid species, in polyploid species with two or more
Table 1 also reveals a range of taxonomic and life cycle ancestral genomes, nuclear C-value always exceeds genome
types. Thus, the total sample of 156 weeds contained 39-1 % size. Moreover, genome size cannot be calculated in any
monocots and 60-9 % dicots, while among 154 species of species whose ploidy level is unknown. In general replicated
known life cycle type, 52'6 % were annuals and 474 % non- genome size equals total 4C nuclear DNA amount divided
annuals. Moreover, IWW and BGW weeds contained 49.5 by ploidy level. This formula gives an accurate estimate in
and 200 % of monocots, and 69 1 and 29-1 % of annuals, taxa with equal sized genomes, but in taxa with genomes of
respectively. IWW and BGW had representatives of 23 and different sizes (e.g. some diploid hybrids and allopolyploids)
29 different families, respectively, while overall 156 weeds it gives a mean genome size. This is close to the actual
Bennett et al.-DNA Amounts in Angiosperm Weeds 127
genome sizes in most, but not all species (e.g. in some taxa weeds, to compare these weeds with other species, is
with bimodal karyotypes). Such values are generally therefore permitted. Such analysis (Table 4) showed that the
accurate enough to allow broad meaningful comparisons, mean 4C DNA amount in 156 weeds (1174pg) was
so the above formula was used to derive mean genome size significantly lower (P < 0-001) than in 2685 other species
values for species of known ploidy level in Table 1. In the (28-13 pg). Testing IWW and BGW separately showed that
present work, nuclear 4C DNA amount is referred to as 4C 4C DNA amounts for both samples were significantly
DNA amount, while mean DNA amount per replicated smaller (P < 0.001) than for other species, irrespective of
genome is referred to as either DNA amount per genome, whether or not the 20 species in common were included in
or, genome size. the analyses.
Analysis of DNA amounts D
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Genome size w
The complete data set comprises 2841 species with known n
total 4C nuclear DNA amounts listed in Table 1, and/or in DNA amount per genome was less variable than 4C loa
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the angiosperm DNA C-values database (Bennett, Cox and nuclear DNA amount. Thus, mean DNA amount per ed
Leitch, 1997). This includes all 156 weeds listed in Table 1 genome in 116 weeds (Table 1) differed only 54-fold, fro
and 2685 other species. If two or more such estimates were ranging from 035 pg in diploid Arabidopsis thaliana to m h
available for a taxon, the value designated as prime by 19-1 pg in tetraploid Ranunculusficaria L. (Table 2). While ttp
Bennett et al. (1977) was used in analysis. the situation for 65 BGW was identical, in 68 IWW genome s://a
size differed only about 32-fold, ranging from 043 pg in c
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RESULTS tetraploid Polygonum persicariaL . to 13-95 pg in tetraploid de
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4C nuclear DNA amount Bperro mgeunso smecea liinn u2s3 L5. 7( Toatbhleer 1)s.p Hecoiews evdiefrf,e mreeda no vDeNr A10 a0m0-ofoulnd,t ic.ou
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4C DNA amounts in 156 weeds (Table 1) differed 143-fold ranging from about 0 11 to 17895 pg in diploid Fritillaria .c
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ranging from 07 pg in diploid Arabidopsis thaliana to davisii Turrill (Table 2). Thus, while mean DNA amount m
100-4 pg in 16-ploid Ranunculus lingua L. (Table 2). While per genome differs considerably among weeds, such vari- /ao
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the situation is identical for the sample of 79 BGW, among ation was very limited and restricted to the lowest 9-4 % of /a
97 IWW 4C DNA amounts differed only 50-fold ranging the range known for other species (Fig. 2A and B). rtic
from 13 pg in diploid Pistia stratiotes L. to 656 pg in The mean genome size was 337 pg for 68 IWW, and le-a
hexaploid Hordeum leporinum var. simulans (Table 2). 3-90 pg for 65 BGW, compared with 1214 pg in 2357 other bs
However, 4C DNA amounts in 2685 other species differed species (Table 2). Tests showed that mean DNA amount per tra
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over 1000-fold, ranging from under 05 to over 500 pg in genome in IWW and BGW did not differ significantly, t/8
2
tetraploid Fritillariaa ssyriaca Baker (Table 2). Thus, while irrespective of whether or not the 17 species in common /s
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4C DNA amounts differed considerably among weeds, such were included in the analyses. Pooling data for these two p
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variation was limited compared with all angiosperms, being types of weeds, to compare weeds with other species, is l_
1
restricted to the lowest 20 % of the range known for other therefore permitted. Such analysis (Table 4) showed that the /1
2
species (Fig. 1 A and B). large difference in mean 4C DNA amount per genome 1/2
The mean 4C nuclear DNA amounts were 10-21 pg for between 116 weeds (379pg) and 2357 other species 11
1
97 IWW and 13-07 pg for 79 BGW compared with 2813 pg (12-14 pg) was highly significant (P < 0.001). Testing IWW 1
9
in 2685 other species (Table 2). The 4C DNA amounts in and BGW separately showed that 4C DNA amounts for b
y
IWW and BGW did not differ significantly, irrespective of both samples were significantly (P < 0001) smaller than for g
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whether or not the 20 species in common were included in other species irrespective of whether or not the 17 species in s
the analysis (Table 3). Pooling data for the two types of common were included in the analyses. t on
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TABLE 2. Mean (followed by the number of species in the sample in parentheses), minimum and maximum and range of 4C m
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nuclear DNA amount and DNA amount per genome (italics)f or sample of weeds, other species, and all angiosperms e
r 2
0
1
Mean Min. Max. Range 8
Sample (pg) (pg) (pg) (Max./Min.)
All weeds 1174 (156) 070 10040 14343
379 (116) 0.35 19.10 54.57
Important world weeds 1021 (97) 130 6560 5056
3.37 (68) 043 1395 32,44
British garden weeds 1307 (79) 070 10040 143-43
3.90 (65) 035 1910 5457
Other species 28.13 (2685) 022 50960 2316-36
1214 (2357) 011 17895 162881
All angiosperms 27'23 (2841) 0.22 50960 2316-36
1175 (2473) 011 17895 1628-81
128 Bennett et al.-DNA Amounts in Angiosperm Weeds
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4C DNA amount (pg) 4C DNA amount (pg) D
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4C DNA amount (pg) 4C DNA amount (pg) p.c
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FIG. 1. The distribution of 4C nuclear DNA amounts for 2841 angiosperm species (A), 156 weeds (B), 97 important world weeds (C) and 79 British m
garden weeds (D). /ao
b
/a
rtic
TABLE 3. Comparison of mean 4C nuclear DNA amount and TABLE 4. Comparisons of mean 4C nuclear DNA amount le
mean DNA amount per genome (italics) in samples of and DNA amount per genome (italics) in samples of weeds -ab
s
important world weeds and British garden weeds and three and other species, and three pairs of sub-samples differing in tra
pairs of sub-samples differing in systematic sub-class, genome systematic sub-class, genome dosage and life cycle type ct/8
dosage and life cycle type 2
/s
Weeds Other species u
p
p
Important world British garden l_
weeds weeds Mean Mean 1
/1
(pg) (pg) P 2
1
Mean Mean /2
(pg) (pg) P Total sample 11-74 (156) 28-13 (2685) *** 11
1
3.79 (116) 12.14 (2357) *** 1
9
Total sample 10-21 (97) 13-07 (79) NS Monocots 14-02 (61) 48-60 (1132) ** b
3.37 (68) 3.90 (65) NS 4.99 (37) 21-20 (968) *** y g
Monocots 14-66 (48) 13-68 (16) NS Dicots 10-27 (95) 13-21 (1553) ** ue
5-16 (30) 4-34 (10) NS 3-23 (79) 5-82 (1389) *** st o
Dicots 5-86 (49) 12-92 (63) * Diploids 7.44 (57) 27-18 (1747) *** n
1-95 (38) 3.38 (55) * 3-72 (57) 13-59 (1747) ** 16
Diploids 7-06 (33) 7.54 (30) NS Polyploids 18-81 (59) 36-23 (610) *** No
3-55 (33) 3.77 (30) NS 3.86 (59) 7.97 (610) *** ve
Polyploids 15-61 (35) 20-10 (35) NS Annuals 10-02 (81) 16-44 (624) *** mb
Annuals 131-.2081 ((6375)) 45..9052 ((2335)) N*S Non-annuals 133-.4728 ((7538)) 363-3581 ((1589717) ) ****** er 20
3.73 (46) 2.40 (20) NS 4-14 (57) 14-93 (1607) *** 18
Non-annuals 8-25 (28) 16-00 (56) NS
2.63 (21) 4-57 (45) NS The number of species for which data were available is given in
parentheses. *** P < 0001, **P < 001.
The number of species in each subsample for which data were
available are given in parentheses. * P < 0-05; NS, non-significant.
taxonomic unit and life cycle type. Thus, the above
comparisons were repeated for three important pairs of
4C nuclear DNA amount, and genome size in sub-groups
subgroups (namely diploids and polyploids; monocots and
A major value of holding species DNA C-values in a dicots; annuals and non-annuals) to test if the large
relational database is the facility to investigate the potential differences between weeds and other species noted above
effects of characters in other fields, such as genetic structure, were broadly representative.
Bennett et al.-DNA Amounts in Angiosperm Weeds 129
The results (Table 3) showed first, that for most (75 %) 4C DNA amounts below 1-7 pg, and ten out of 11 lowest are
sub-groups IWW did not differ significantly from BGW diploids. Both IWW and BGW independently show this
irrespective of whether or not the three-20 species in trend. Among IWW, eight out of ten (80%) and 15 out of
common were included in the analyses. Two exceptions for 20 (75%) species with the lowest C-values are diploids,
4C nuclear DNA amount were annuals where the mean for while IWW with large C-values are predominantly poly-
BGW (595 pg) was significantly lower (P < 005) than for ploids (e.g. 80 % of the top ten). Similarly, the species with
IWW (11-08 pg), and dicots where the mean for BGW the six lowest C-values among BGW were all diploids, while
(1292 pg) was significantly higher (P < 005) than for IWW nine out of ten with the highest C-values were polyploids.
(5.86 pg). The one exception for DNA amount per genome Interestingly, the species with the highest 4C value among
was dicots where the mean for IWW (195 pg) was 156 weeds also had the highest ploidy level (16x Ranunculus
significantly lower (P < 005) than for BGW (3.38 pg). lingua), while the three highest 4C values included two out
D
Apart from these exceptions, which must be tested sep- of the four species with the three highest ploidy levels (7x, o
w
arately, it is permissible to pool the data for both types of 8x and 16x). Together the present results suggest that: (1) nlo
weeds. Testing genome size in dicot IWW and BGW the frequency of polyploids may increase with 4C value in ad
e
separately showed that both IWW (P < 0001) and BGW weeds; and (2) there may be a maximum cut-off point for d
(P < 005) were significantly lower than other dicot species. increasing 4C DNA amount, above which weed species are fro
m
Similarly, testing 4C nuclear DNA amounts for annual all polyploids. h
IWW and BGW separately showed that both were sig- ttp
DISCUSSION s
nificantly lower (P < 0001) than other annual species. ://a
However, testing 4C nuclear DNA amounts for dicot IWW Only about 200 species are recognized as important weeds ca
d
and BGW separately showed that while IWW (P < 0001) on a world-wide basis (Holm et al., 1997), and DNA e
m
were significantly lower than other dicot species, BGW were estimates are now known for 97 IWW species (Table 1). ic
not. Moreover, Holm et al., (1977) listed the world's 18 worst .ou
p
Further tests showed that while 25 diploid dicot BGW agricultural weeds of which DNA estimates for 17 of them .c
o
(7 50 pg) were significantly lower (P < 0.05) than 1107 other [including first estimates for ten species (i.e. > 55 %) not m
diploid dicots (1287 pg), only 30 polyploid dicot BGW previously listed elsewhere] are given in Table 1. The present /ao
b
(1967 pg) were not significantly different (P = 0'53) from work giving the first 4C nuclear DNA estimates for 39 IWW /a
282 polyploid dicot other species (1683 pg), among several (approx. 20%) has therefore significantly increased our rtic
le
sub-groups tested. Second, that perennial polyploid BGW knowledge of the character in this important class of weeds. -a
b
of one family (Ranunculaceae), the most represented among Other key texts together recognized 135 species as British s
BGW but unrepresented in the IWW sample, produced this garden weeds, and 4C nuclear DNA amounts are listed for trac
atypical result. Repeating the tests while omitting the eight 79 of these (Table 1), including three first estimated in the t/8
2
polyploid Ranunculaceae transformed the previously sig- present work. With data now available for 97 (approx. /s
u
nificant results for dicots and their sub-groups to non- 50 %) and 79 (approx. 58 %) species, respectively, IWW and pp
significance. This result shows how one atypical group can BGW are both well represented in the angiosperm DNA C- l_1
distort results. Notwithstanding this one interesting ex- values database, with samples large enough to allow /12
1
ception, the above results show that pooling results for the meaningful conclusions regarding nuclear DNA amount /2
1
two weed samples is generally permissible. Moreover, testing and genome size in these two samples of weeds. Analysis of 1
1
the results for weeds against other species shows clearly that the above results showed first that DNA amounts in the two 19
the overall difference was repeated for all the subgroups samples of different types of weeds are so similar (Table 4) by
tested (Table 4). Thus, weeds generally had highly sig- as to permit pooling their data. As the results for two gu
e
nificantly smaller mean 4C DNA amounts and genome sizes different types of weeds agree, they may also provide useful s
than other species, both in IWW and BGW, and in all the indications about these characters in a broader range of t on
major subgroups tested, i.e. diploids and polyploids, weed types. 16
N
monocots and dicots, annuals and non-annuals. Moreover, o
v
comparing 4C DNA amounts for all 33 families with data e
Evidence for selection against high C-value and genome m
for weeds and other species showed that values for weeds b
size in weeds e
are restricted to the lower 50 % of the range known for the r 2
family in 64 % of cases. Thus, the tendency for DNA values Interestingly, analysis of the above results showed that 01
8
to be lower in weeds than in other species is not limited to, DNA amounts for IWW and BGW, either pooled together
and determined by, a particular sub-group, but represents a or as two separate samples (Table 2), showed similar highly
broadly based phenomenon involving a wide range of plants significant differences from those for other species. For
of differing genetic structure, taxonomic group, or life form. example, the mean 4C nuclear DNA amount in 156 weeds
Further examination of the results for sub-groups may (11 74 pg) was significantly lower (P < 0001) than for 2685
provide a useful insight into a cause of this major difference. other species (28.13 pg), while the range of 4C nuclear DNA
Thus, among species of known ploidy level, all 11 weed amounts for such species (070-1004 pg) was much smaller
species whose 4C DNA amounts exceed 40 pg are polyploids than for other species (0.22-509.6 pg), and occupied only
(Table 1), as are 17 out of 18 species whose 4C DNA the bottom 20 % of the range for other angiosperms (Fig. 1).
amounts exceed 25 pg. This differs from the situation Similarly, mean 4C DNA amount per genome in 116 weeds
among weeds with small genomes, as all seven species with (3.79 pg) was significantly lower (P < 0001) than in 2357
130 Bennett et al.-DNA Amounts in Angiosperm Weeds
450 40
o~ 4[00 A , 35- B
·a3, 150 ' 30-
100
' 25-
150
0 o 20-
a 2200
°V 15
2 a.
100 10
50
0 50 100 150 20C 0 50 100 150 200
DNA amount per genome (pg) DNA amount per genome (pg)
D
o
w
n
30 CC on D load
Wa 25 ed
U4ato)o 20 from
.0W0r 15 0 10 http
s
zaZ 105 a 5 ://aca
-I ., . d
e
m
0 50 100 150 200 0 50 100 150 200 ic.o
DNA amount per genome (pg) DNA amount per genome (pg) up
.c
FIG. 2. The distribution of DNA amount per genome for 2473 angiosperm species (A), 116 weeds (B), 68 important world weeds (C) and 65 British o
m
garden weeds. /a
o
b
/a
other species of known ploidy level (12-14 pg), while the negative relationships for 68 IWW (P < 0-01; r = -0'935), rtic
range of genome sizes in these weeds (0-35-19 1 pg) was and for 65 BGW (P < 0-001; r = -0987) respectively, are le-a
much smaller than in non-weeds (0 11-17895 pg). Genome shown in Fig. 4B and C. bs
sizes for weeds occupied only the bottom 107% of the If the ability to be an important weed is related with DNA trac
range for angiosperms (Fig. 2). It is concluded, therefore, amount, so that such species tend to have a smaller DNA t/8
2
that strong selection exists for low nuclear DNA amount amount per nucleus and per genome than other species, then /s
u
and genome size in many species recognized as weeds. the very worst weeds might be expected to have smaller pp
Alternatively small nuclear DNA amount and genome size DNA amounts than less troublesome weeds. Analysis shows l_1
may be useful preadaptations allowing species to become that this expectation is also met. Holm et al. (1977) listed the /12
1
weeds. Indeed, both may be true. world's 18 worst agricultural weeds ranked in order of /2
1
If the hypothesis that there is strong selection for reduced importance, and Table I includes DNA amounts for 17 of 1
1
nuclear DNA amount and genome size in many species them. Mean 4C nuclear DNA amount increases steadily for 19
recognized as weeds is correct, then negative relationships the world's top five, ten, 17 out of 18, and 97 out of about by
between these characters and the proportions of species 200 worst weeds (374, 668, 885 and 10-71 pg). The data for gu
e
recognized as successful weeds are expected. Analysis shows mean genome size shows a similar trend (15, 143, 24 and s
that these expectations are met. Figure 3 A shows the highly 337 pg, respectively). t on
significant negative correlation (P < 0001; r = -0986) Together the above results show that the probability of 16
between the numbers of weed species in Table 1 (expressed being a recognized weed decreases with both increasing 4C No
as a percentage), among the 2719 species with 4C nuclear total nuclear DNA amount and increasing mean genome ve
m
DNA amounts of 100-4 pg (the maximum for 156 weeds) size, reaching zero at cut-off values just above 100 pg for the b
e
divided into five groups of 544 or 543 species, and a sixth former, and just above 19 pg for the latter. While many r 2
smaller group of 122 species whose 4C nuclear DNA species have DNA amounts exceeding these values, it seems 01
8
amounts exceed 1004 pg, ranked by increasing 4C nuclear they may be nucleotypically determined, obligate non-
DNA amount in the angiosperm DNA C-values database. weeds of these two types. Whereas most life style options are
The proportion of weed species in the lowest group is 9.6 %, open to species with small genomes, many are progressively
but this consistently decreased for each successively higher shut off as C-value and genome size increase. This was
group, reaching zero for group 6 above 100-4 pg. Figure 3 B previously recognized for ephemerals and annuals (Bennett,
and C shows similar significant negative relationships for 1987). It is now suggested to be true for some types of weeds
both 97 IWW (P < 001; r = -0961), and 79 BGW (P < also.
0-01; r = -0965). A corresponding plot (Fig. 4A) of mean While mean 4C nuclear DNA amount in 156 weeds
genome size for 116 weed species with known mean genome (1174 pg) is significantly lower (P < 0-001) than for 2685
size (Table 1) shows a similar significant negative re- other species (27.13 pg), this difference might reflect a major
lationship (P < 0-001; r =-0-977). Separate significant effect of one atypical sub-sample, like that noted above for