Complex pattern of genome size variation in a polymorphic member of the Asteraceae

Aim  Although divergences in nuclear DNA content among different species within a genus are widely acknowledged, intraspecific variation is still a somewhat controversial issue. The aim of this study was to assess genome size variation in the polymorphic species Picris hieracioides L. (Asteraceae) and to search for potential interpretations of the size heterogeneity.
Location  Europe.
Methods  The genome sizes of 179 plants of P. hieracioides collected from 54 populations distributed across 10 European countries were determined by propidium iodide flow cytometry. Differences in nuclear DNA content were confirmed in simultaneous analyses.
Results  2C-values (population means) at the diploid level varied from 2.26 to 3.11 pg, spanning a 1.37-fold range. The variation persisted even after splitting the whole data set into two recently distinguished morphotypes (i.e. the 'Lower altitude' type and the 'Higher altitude' type) that possess significantly different nuclear DNA contents. Cluster analysis revealed the presence of three major groups according to genome size, which exhibited a particular geographical pattern. Generally, the genome size of both morphotypes increased significantly from south-west to north-east. A new cytotype, DNA triploid, was found for the first time.
Main conclusions  High intraspecific variation in the amount of nuclear DNA in P. hieracioides correlates with the extensive morphological variation found within the taxon. Despite the complex pattern that was presented, genome size variants were non-randomly distributed and reflected palaeovegetation history. We suggest that the complex evolutionary history of P. hieracioides (e.g. the existence of several cryptic lineages with different levels of cross-interactions) is the most plausible explanation for the observed heterogeneity in genome size.     

Genome size and base composition in Medicago sativa and M. truncatula species.

The genome size (1C value) and base composition of 14 ecotypes of two species of tetraploid and diploid Medicago have been assessed by flow cytometry. These parameters vary both between and within species. The diploid annual Medicago truncatula Gaertn. had the smallest genome of the group studied (which also covered M. sativa L. subsp. sativa, M. sativa L. subsp. caerulea (Less. ex Ledeb.) Schmalh., M. sativa L. subsp. quasifalcata Sinsk., M. sativa L. subsp. x varia (Martyn) Arcangeli; however, its ecotypes revealed substantial intraspecific variation. The smallest M. truncatula genome observed was ecotype 108-1 with 1C = 0.49 pg and 38.1% GC and the largest was Jemalong with 1C = 0.57 pg and 38.6% GC. The degree of polysomaty in these Medicago was low, although in some tissues the frequency of cells with 4C nuclei reached 50%.     

Genome size and genome evolution in diploid Triticeae species.

One of the intriguing issues concerning the dynamics of plant genomes is the occurrence of intraspecific variation in nuclear DNA amount. The aim of this work was to assess the ranges of intraspecific, interspecific, and intergeneric variation in nuclear DNA content of diploid species of the tribe Triticeae (Poaceae) and to examine the relation between life form or habitat and genome size. Altogether, 438 plants representing 272 lines that belong to 22 species were analyzed. Nuclear DNA content was estimated by flow cytometry. Very small intraspecific variation in DNA amount was found between lines of Triticeae diploid species collected from different habitats or between different morphs. In contrast to the constancy in nuclear DNA amount at the intraspecific level, there are significant differences in genome size between the various diploid species. Within the genus Aegilops, the 1C DNA amount ranged from 4.84 pg in A. caudata to 7.52 pg in A. sharonensis; among genera, the 1C DNA amount ranged from 4.18 pg in Heteranthelium piliferum to 9.45 pg in Secale montanum. No evidence was found for a smaller genome size in annual, self-pollinating species relative to perennial, cross-pollinating ones. Diploids that grow in the southern part of the group's distribution have larger genomes than those growing in other parts of the distribution. The contrast between the low variation at the intraspecific level and the high variation at the interspecific one suggests that changes in genome size originated in close temporal proximity to the speciation event, i.e., before, during, or immediately after it. The possible effects of sudden changes in genome size on speciation processes are discussed.     

Intraspecific genome size variation and morphological differentiation of Ranunculus parnassifolius (Ranunculaceae), an Alpine–Pyrenean–Cantabrian polyploid group

The aim of this study was to assess genome size variation and multivariate morphometric analyses to ascertain cytotype distribution patterns and the morphological differentiation within the Ranunculus parnassifolius group in the Pyrenees and the Alps. Although divergences in nuclear DNA content among different species within a genus are widely acknowledged, intraspecific variation is still a somewhat controversial issue. Holoploid and monoploid genome sizes (C‐ and Cx‐values) were determined using propidium iodide flow cytometry in 125 plants of R. parnassifolius s.l. distributed across four European countries. Three different DNA ploidy levels were revealed in the study area: diploid (2n ～ 2x, 57.14%), triploid (2n ～ 3x, 1.19%), and tetraploid (2n ～ 4x, 41.67%). The mean population 2C‐values ranged from 8.15 pg in diploids to 14.80 pg in tetraploids, representing a ratio of 1 : 1.8. Marked intraspecific/interpopulation differences in nuclear DNA content were found. Diploid populations prevail in the Pyrenees, although tetraploid cytotypes were reported throughout the distribution area. In general, mixed‐cytotype populations were not found. The Spearman correlation coefficient did not reveal significant correlations between genome size and altitude, longitude, or latitude. Morphometric analyses and cluster analyses based on genome size variation revealed the presence of three major groups, which exhibited a particular biogeographical pattern. A new cytotype, DNA triploid, was found for the first time. Tetraploid populations showed constant nuclear DNA levels, whereas diploid populations from the Pyrenees, in which introgressive hybridization is suggested as a presumable trigger for genome size variation, did not. Scenarios for the evolution of geographical parthenogenesis in R. parnassifolius s.l. are discussed. Finally, the different levels of effectiveness between plant and animal reference standards are analysed. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 251–271.     

Repetitive sequences: cause for variation in genome size and chromosome morphology in the genus Oryza

Large variation in genome size as determined by the nuclear DNA content and the mitotic chromosome size among diploid rice species is revealed using flow cytometry and image analyses. Both the total chromosomal length (r_0.939) and the total chromosomal area (r_0.927) correlated well with the nuclear DNA content. Among all the species examined, Oryza australiensis (E genome) and O. brachyantha (F genome), respectively, were the largest and smallest in genome size. O. sativa (A genome) involving all the cultivated species showed the intermediate genome size between them. The distribution patterns of genome-specific repetitive DNA sequences were physically determined using fluorescence in situ hybridization (FISH). O. brachyantha had limited sites of the repetitive DNA sequences specific to the F genome. O. australiensis showed overall amplification of genome-specific DNA sequences throughout the chromosomes. The amplification of the repetitive DNA sequences causes the variation in the chromosome morphology and thus the genome size among diploid species in the genus Oryza.     

A biosystematic investigation of the Asplenium lepidum complex

Twenty-nine populations of three closely related taxa, Asplenium lepidum, A. haussknecktii and A. samarkandense , have been raised together in cultivation. There is considerable inter-population variation, and the ranges of variation of the three species overlap. Cytogenetic investigation of artificial hybrids has shown that the three taxa are all allotetraploid and that they share the same diploid parents, A. aegaeum and A. ruta-muraria subsp. dolomiticum. Analysis of F1 and F2 hybrids between different populations indicates high inter-fertility.
The aggregate is re-defined as a single polymorphic species (A. lepidum) consisting of two subspecies (subsp. lepidum and subsp. haussknechtii) distinguished primarily by gland and sporangial characters. A. samarkandense is reduced to a variety of subsp. haussknechtii. It is suggested that the two subspecies have had separate geographical origins from different morphological variants of the same diploid parents, but the polymorphic nature of the aggregate is principally due to divergent evolution in small populations isolated on limestone mountains in southern Europe and western Asia.     

Variation of Nuclear DNA Content in the Biflorus Species of Lonchocarpus (Leguminosae)

This represents the first study of nuclear DNA content in alarge sample (135 spp.) from a tropical arboreal genus, in whicha large proportion of the species were examined (42 spp., 31.1%).Somatic chromosome numbers and 4C-DNA values for 51 taxa ofLonchocarpus are reported. All taxa were diploid with 2 n =22,but their DNA content ranged from 1.92 to 2.86 pg 4C nucleus,corresponding to a 48.95% variation in genome size. In the 74collections studied, no correlation was observed between DNAcontent and habitat altitude. Variation in nuclear DNA contentwas analysed at the level of genus, subgenus, section and subsection.Variation in genome size was also studied within some species,either among widely separated populations or among differentintraspecific taxa. Very little variation in genome size wasdetected between populations, subspecies, and varieties of thesame species. The taxonomic implications of variation in nuclearDNA content are discussed.Copyright 2000 Annals of Botany Company Lonchocarpus (Leguminosae), DNA content, chromosome number.     

A Morphometric Study of Populations of the Centaurea jacea Complex (Asteraceae) in Belgium

Abstract: The Centaurea jacea aggregate is a polymorphic polyploid complex whose taxonomic treatment is still controversial. A numerical taxonomic approach was applied to 394 individuals of known ploidy level, from 19 populations, based on the main diagnostic characters proposed in earlier revisions. Populations from xeric grasslands were not considered. Principal Component Analysis shows that variation within the complex is continuous. UPGMA Cluster Analysis based on population means supports the recognition of three groups of populations. However, the limits between these groups are blurred to a considerable extent due to extensive within-population polymorphism. It is argued that the Belgian populations of Centaurea jacea occurring in mesic grasslands should be treated as a single species, with three subspecies. The two extremes of the morphological gradient can be referred to as C. jacea subsp. jacea and C. jacea subsp. nigra, with C. jacea subsp. pratensis occupying an intermediate position. Most populations from Belgium are tetraploid, a diploid chromosome number being found only in populations of C. jacea subsp. nigra from the Ardennes massif. On average, diploids grow at higher altitude and on more acidic soils than tetraploids. Finally, a key to the three subspecies is provided.     

Variation in chromosomal DNA associated with the evolution of Arachis species.

The 2C nuclear DNA amounts were determined for 99 accessions, representing 23 Arachis species from 8 of 9 taxonomic sections, and two synthetic amphidiploids. Mean 2C DNA amounts varied by 15.20%, ranging from 10.26 to 11.82 pg, between accessions of Arachis hypogaea (2n = 4x = 40). Nuclear DNA content variation (5.33-5.91 pg) was also detected among Arachis duranensis (2n = 2x = 20) accessions. The intraspecific variation in the two species may have resulted from indirect selection for favourable genome sizes in particular environmental conditions. The accessions belonging to A. hypogaea ssp. hypogaea (mean value 11.27 pg) with longer life cycle had significantly larger mean DNA content than the accessions of A. hypogaea ssp. fastigiata (mean value 10.97 pg). For 20 diploid (2n = 2x = 20) species of the genus, 2C nuclear DNA amounts ranged from approximately 3 to 7 pg. The diploid perennial species of section Arachis have about 12% more DNA than the annual species. Comparisons of DNA amounts show that evolutionary rating is not a reliable guide to DNA amounts in generic sections of the genus; lower DNA values with evolutionary advancement were found in sections Heteranthae and Triseminatae, but the same was not true for sections Arachis and Caulorrhizae. Similarly, there is evidence of significant differences in DNA content between 4 ancient sections (Procumbentes, Erectoides, Rhizomatosae, and Extranervosae) of the genus. The occurrence of genome size plasticity in both A. duranensis and A. hypogaea provides evidence that A. duranensis could be one of the diploid progenitors of A. hypogaea. The DNA content in the two synthetic amphidiploids corresponded to the sum value estimated for parental species. Key words : Arachis species, genome size, Arachis hypogaea, Arachis duranensis, intraspecific variation.     

Nuclear DNA content variation among perennial taxa of the genus Cyanus (Asteraceae) in Central Europe and adjacent areas

The genome size of 265 plants and the GC content of 126 plants from 63 populations of the Cyanus triumfetti and Cyanus montanus groups, collected across the Carpathians, Pannonia, Bohemian Massif, and Western and Dinaric Alps were determined by PI and DAPI flow cytometry. Variation of the nuclear DNA content among homoploid species, and intraspecific and interpopulation variation were confirmed in simultaneous analyses. The 2C-value at the diploid level (the C. triumfetti group) varied from 2.53 for Cyanus dominii subsp. sokolensis to 3.06?pg for C. triumfetti s.s. (1.21-fold range). At the tetraploid level (the C. montanus group), the 2C-value varied from 5.19 for Cyanus mollis to 5.84?pg for C. montanus (1.13-fold range). High intraspecific and interpopulation variation in the amount of nuclear DNA in the C. triumfetti group correlates with the extensive morphological variation found in this group. Significant between-species differences in genome size indicate that this attribute may be used as a supportive taxonomic marker for both of the groups studied. The GC content varied by 2.93?%, from 39.46?% for “Cyanus axillaris” to 40.61?% for Cyanus adscendens; this character is of no value for taxonomic purposes. Genome size of the studied populations is significantly higher in southern parts of the distribution area and at higher elevations. Plants with smaller genomes tend to occur in dry areas at low altitudes with high diurnal and annual temperature oscillations. The GC content of the populations studied is significantly correlated with longitude, increasing from east to west; and plants with GC-rich genomes are concentrated in the coldest areas with low minimum temperatures.     

Origin of the H genome in StH-genomic Elymus species based on the single-copy nuclear gene DMC1

Previous studies have suggested that the H haplome in Elymus could originate from different diploid Hordeum species, however, which diploid species best represent the parental species remains unanswered. The focus of this study seeks to pinpoint the origin of the H genome in Elymus. Allopolyploid Elymus species that contain the StH genome were analyzed together with diploid Hordeum species and a broad sample of diploid genera in the tribe Triticeae using DMC1 sequences. Both parsimony and maximum likelihood analyses well separated the American Hordeum species, except Hordeum brachyantherum subsp. californicum, from the H genome of polyploid Elymus species. The Elymus H-genomic sequences were formed into different groups. Our data suggested that the American Horedeum species, except H. brachyantherum subsp. californicum, are not the H-genomic donor to the Elymus species. Hordeum brevisubulatum subsp. violaceum was the progenitor species to Elymus virescens, Elymus confusus, Elymus lanceolatus, Elymus wawawaiensis, and Elymus caninus. Furthermore, North American H. brachyantherum subsp. californicum was a progenitor of the H genome to Elymus hystrix and Elymus cordilleranus. The H genomes in Elymus canadensis, Elymus sibiricus, and Elymus multisetus were highly differentiated from the H genome in Hordeum and other Elymus species. The H genome in both North American and Eurasian Elymus species was contributed by different Hordeum species.     

Divergent genome sizes reflect the infrafamilial subdivision of the neotropical woody Marcgraviaceae

Neotropical Marcgraviaceae comprise about seven genera and 130 species of lianas and shrubs. They predominantly occur in lowland or montane rainforests and are characterized by a variety of pollination systems. Early classifications subdivided Marcgraviaceae into subfamilies Marcgravioideae and Noranteoideae, a concept supported by molecular data. Using flow cytometry and chromosome numbers, we investigated the role of genome size and polyploidization in the evolution of Marcgraviaceae and how genome sizes are distributed between the proposed infrafamilial groups. To do this we determined genome sizes and chromosome counts for six genera and 22 species for the first time. Our study supports the subfamilial classification of the family, revealing contrasting genome sizes in Noranteoideae (2C = 5.5–21.5 pg) and Marcgravioideae (2C = 2.3–6.2 pg). Polyploidy is considered to be the main source of genome size variation as in each subfamily the higher nuclear DNA amounts were associated with higher ploidy. In addition, genome size changes independent of polyploidy were also observed in some genera, suggesting an additional role for changes in repetitive DNA abundance in the evolution of Marcgraviaceae. A high chromosome base number (x = 18; 2n = 36 to ～70) points to an undetected lower diploid level or to palaeopolyploidy. Marcgraviaceae show a remarkable (nine‐fold) variation in genome size, and several Noranteoideae have genome sizes among the highest reported for tropical woody angiosperms worldwide. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 1–14.     

Genome sizes of Eucomis L’Hér. (Hyacinthaceae) and a description of the new species Eucomis grimshawii G.D.Duncan & Zonneveld

Nuclear genome size, as measured by flow cytometry with propidium iodide, was used to investigate the relationships within the genus Eucomis L’Hér. (Hyacinthaceae). Most species of Eucomis have the same basic chromosome number, x = 15. However, the somatic DNA 2C-value (2C) is shown to range from 21 to 31 pg for the diploids. The largest genome contains roughly 10¹⁰ more base pairs than the smallest. Genome sizes are evaluated here in combination with available morphological and geographical data. Therefore, the taxonomy proposed here is not based on genome size alone. The genus Eucomis, as here determined, has 12 species. These can be divided into two groups: mainly dwarf diploid species and large-sized, tetraploid species. A small diploid plant, Eucomis (autumnalis subsp.) amaryllidifolia, is restored to species status, as a diploid subspecies seems incongruent with an allotetraploid Eucomis autumnalis. Moreover, as a diploid it is separated reproductively from the allotetraploid E. autumnalis. A new diploid species that has the lowest C value, E. grimshawii, is described here. On the basis of DNA content and other morphological characters, possible parents are suggested for all tetraploid species. Nuclear DNA content as measured by using flow cytometry may conveniently be used to produce systematic data. It is applicable even in dormant bulbs or sterile plants for the monitoring of the trade in bulbous species.     

Genome size variation in Central European species of Cirsium (Compositae) and their natural hybrids

BACKGROUND AND AIMS: Nuclear DNA amounts of 12 diploid and one tetraploid taxa and 12 natural interspecific hybrids of Cirsium from 102 populations in the Czech Republic, Austria, Slovakia and Hungary were estimated. METHODS: DAPI and PI flow cytometry were used. KEY RESULTS: 2C-values of diploid (2n = 34) species varied from 2.14 pg in C. heterophyllum to 3.60 pg in C. eriophorum (1.68-fold difference); the 2C value for the tetraploid C. vulgare was estimated at 5.54 pg. The DNA contents of hybrids were located between the values of their putative parents, although usually closer to the species with the smaller genome. Biennial species of Cirsium possessed larger nuclear DNA amounts than their perennial relatives. Genome size was negatively correlated with Ellenberg's indicator values for continentality and moisture and with eastern limits of distribution. A negative relationship was also detected between the genome size and the tendency to form natural interspecific hybrids. On the contrary, C-values positively corresponded with the spinyness (degree of spinosity). AT frequency ranged from 48.38 % in C. eriophorum to 51.75 % in C. arvense. Significant intraspecific DNA content variation in DAPI sessions was detected in C. acaule (probably due to the presence of B-chromosomes), and in tetraploid C. vulgare. Only the diploid level was confirmed for the Pannonian C. brachycephalum, generally considered to be tetraploid. In addition, triploidy was discovered for the first time in C. rivulare. CONCLUSIONS: Considerable differences in nuclear DNA content exist among Central European species of Cirsium on the diploid level. Perennial soft spiny Cirsium species of wet habitats and continental distributions generally have smaller genomes. The hybrids of diploid species remain diploid, and their DNA content is smaller than the mean of the parents. Species with smaller genomes produce interspecific hybrids more frequently.     

Diversity and evolution of the Hordeum murinum polyploid complex in Algeria

Population diversity and evolutionary relationships in the Hordeum murinum L. polyploid complex were explored in contrasted bioclimatic conditions from Algeria. A multidisciplinary approach based on morphological, cytogenetic, and molecular data was conducted on a large population sampling. Distribution of diploids (subsp. glaucum) and tetraploids (subsp. leporinum) revealed a strong correlation with a North-South aridity gradient. Most cytotypes exhibit regular meiosis with variable irregularities in some tetraploid populations. Morphological analyses indicate no differentiation among taxa but high variability correlated with bioclimatic parameters. Two and three different nuclear sequences (gene coding for an unspliced genomic protein kinase domain) were isolated in tetraploid and hexaploid cytotypes, respectively, among which one was identical with that found in the diploid subsp. glaucum. The tetraploids (subsp. leporinum and subsp. murinum) do not exhibit additivity for 5S and 45S rDNA loci comparative with the number observed in the related diploid (subsp. glaucum). The subgenomes in the tetraploid taxa could not be differentiated using genomic in situ hybridization (GISH). Results support an allotetraploid origin for subsp. leporinum and subsp. murinum that derives from the diploid subsp. glaucum and another unidentified diploid parent. The hexaploid (subsp. leporinum) has an allohexaploid origin involving the two genomes present in the allotetraploids and another unidentified third diploid progenitor.     

Cytological studies inCyclamen subg.Cyclamen (Primulaceae)

Although theCyclamen subg.Cyclamen spp. are morphologically variable, previous studies suggest a superficial cytological uniformity. New chromosome numbers and an indication of karyotypic instability are reported inC. hederifolium andC. africanum that reveal that the cytology of the subgenus is more complicated than previous accounts suggest. The possible significance of these phenomena is discussed. The diploid status ofC. purpurascens has been confirmed and a distinction between the three diploid karyotypes has been described. The cytological variation may help to explain the well documented morphological variation exhibited in these species.     

Phylogenetic inferences in Avena based on analysis of FL intron2 sequences

The development and application of molecular methods in oats has been relatively slow compared with other crops. Results from the previous analyses have left many questions concerning species evolutionary relationships unanswered, especially regarding the origins of the B and D genomes, which are only known to be present in polyploid oat species. To investigate the species and genome relationships in genus Avena, among 13 diploid (A and C genomes), we used the second intron of the nuclear gene FLORICAULA/LEAFY (FL int2) in seven tetraploid (AB and AC genomes), and five hexaploid (ACD genome) species. The Avena FL int2 is rather long, and high levels of variation in length and sequence composition were found. Evidence for more than one copy of the FL int2 sequence was obtained for both the A and C genome groups, and the degree of divergence of the A genome copies was greater than that observed within the C genome sequences. Phylogenetic analysis of the FL int2 sequences resulted in topologies that contained four major groups; these groups reemphasize the major genomic divergence between the A and C genomes, and the close relationship among the A, B, and D genomes. However, the D genome in hexaploids more likely originated from a C genome diploid rather than the generally believed A genome, and the C genome diploid A. clauda may have played an important role in the origination of both the C and D genome in polyploids.     

Genome size stability across Eurasian Chenopodium species (Amaranthaceae)

Tremendous interspecific genome size variation is a well known phenomenon, whereas genome size within a species is supposed to be exceptionally stable and thus useful as a taxonomic trait. Using DAPI flow cytometry, we tested the stability of genome size in various representatives of Chenopodium s.s. (Amaranthaceae) across a broad geographical range (from Portugal to eastern Russia) in Eurasia. We sampled 1977 Chenopodium individuals of four different ploidies (di‐, tetra‐, hexa‐ and decaploids) from 347 populations. Intraspecific relative genome size variation was low, ranging from 2.0% in C. probstii to 7.7% in C. album, even in the species with broad distributions. We distinguished 12 homogeneous relative genome size groups among the 17 Chenopodium spp. tested. Genome size is useful for distinguishing certain morphologically similar groups of species such as C. suecicum/C. album, C. vulvaria/C. pamiricum–C. iljinii/C. sosnowskyi/C. karoi. Due to its genome size stability, the cosmopolitan species C. album can be used as an alternative internal standard in flow‐cytometric analyses with the additional advantages of annual life cycle, self‐compatibility and common occurrence all over the world. Finally, we did not detect any sign of hybridization between Chenopodium spp. of different ploidies.     

The relationship between flower size, inbreeding coefficient and inferred selfing rate in British Euphrasia species

The genus Euphrasia in Britain comprises a taxonomically complex group of self-compatible, morphologically similar, hemi-parasitic, annual plant species of high conservation importance. The 19 diploid and tetraploid taxa currently recognised show striking variation in flower size. The objective of this paper is to determine whether a relationship exists between flower size and breeding system within Euphrasia. Following a survey of flower size variation among the 19 taxa, seven diploid populations, encompassing a broad range of flower sizes, were selected for detailed study. Four nuclear microsatellite loci were used to estimate the inbreeding coefficient Fis within each population. Fis values varied from to 0.17-0.77 and showed a significant, negative correlation with flower size. These results are best explained as the consequence of variation in selfing rate among the Euphrasia populations, with selfing rate increasing as flower size decreases. The potential factors influencing breeding system evolution in Euphrasia are discussed, together with the role of autogamy in generating taxonomic complexity and facilitating lineage differentiation within the genus.     

Genome size and karyotype length in some interstitial polychaete species of the genus Ophryotrocha (Dorvilleidae).

Interstitial polychaetes of the genus Ophryotrocha are very small, progenetic, and morphologically very similar. These worms have been widely used in evolutionary biology and sexuality studies. To have a better insight into the karyological evolution of this genus, we measured the total karyotypic length and the 2C nuclear DNA content of the nine best-known species of this genus. No interspecific differences were observed in karyotypic lengths, apart from that of O. gracilis, which was significantly greater than the karyotypic length of five of the nine species. The genome size (i.e., 1C DNA content calculated from 2C DNA content) in eight of the nine species is about 0.4 pg, irrespective of the chromosome number. A group of four gonochoric and morphologically indistinguishable species, with 2n = 6 metacentric chromosomes, appears to be heterogeneous with regard to its DNA content, because one of the species, O. macrovifera, has a genome twice the size of that of the other three species. A hermaphroditic species, O. hartmanni, has a genome three times that size. No correlation has been observed between genome size and body size, egg cell diameter, or time interval from egg fertilization to sexual maturity. The basic genome size of 0.4 pg is among the lowest recorded in invertebrates. Hypotheses about selective pressures that maintain such a low amount of nuclear DNA in this genus are discussed.