Polyploid Races Research Articles

Diversity of polyploid races in the family Lumbricidae

Drawing on the results of research and published data in this paper, we have examined the rate of occurrence of polyploids (genome mutation) in Lumbricidae. As is shown, the frequency of occurrence of polyploid karyotypes (46) almost equals that of diploids (52). Comparing the geographic distribution of these forms, exemplified by Eisenia nordenskioldi (Eisen), the following tendency become evident: polyploid races, as a rule, tend to occupy the margins of the species range. In cases of sympatry of diploids and polyploids of the same species, they tend to take up different ecological niches. Hence, genetic diversity results in species diversity, with polyploid races of the same species being, in fact, sibling species.

Evolutionary implications of allozyme and RAPD variation in diploid populations of dioecious buffalograssBuchloë dactyloides

AbstractBuffalograss,Buchloë dactyloides, is widely distributed throughout the Great Plains of North America, where it is an important species for rangeland forage and soil conservation. The species consists of two widespread polyploid races, with narrowly endemic diploid populations known from two regions: central Mexico and Gulf Coast Texas. We describe and compare the patterns of allozyme and RAPD variation in the two diploid races, using a set of 48 individuals from Texas and Mexico (four population samples of 12 individuals each). Twelve of 22 allozyme loci were polymorphic, exhibiting 35 alleles, while seven 10‐mer RAPD primers revealed 98 polymorphic bands. Strong regional differences were detected in the extent of allozyme polymorphism: Mexican populations exhibited more internal gene diversity (He= 0.20, 0.19) than did the Texan populations (He= 0.08, 0.06), although the number of RAPD bands in Texas (n= 62) was only marginally smaller than in Mexico (n= 68).F‐statistics for the allozyme data, averaged over loci, revealed strong regional differentiation (meanFRT=+ 0.30), as well as some differentiation among populations within regions (meanFPR=+ 0.09). In order to describe and compare the partitioning of genetic variation for multiple allozyme and RAPD loci, we performed an Analysis of Molecular Variance (AMOVA). AMOVA for both allozyme and RAPD data revealed similar qualitative patterns: large regional differences and smaller (but significant) population differences within regions. RAPDs revealed greater variation among regions (58.4% of total variance) than allozymes (45.2%), but less variation among individuals within populations (31.9% for RAPDs vs. 45.2% for allozymes); the proportion of genetic variance among populations within regions was similar (9.7% for RAPDs vs. 9.6% for allozymes). Despite this large‐scale concordance of allozyme and RAPD variation patterns, multiple correlation Mantel techniques revealed that the correlations were low on an individual by individual basis. Our findings of strong regional differences among the diploid races will facilitate further study of polyploid evolution in buffalograss.

RAPD variation within and among natural populations of outcrossing buffalograss [Buchloë dactyloides (Nutt.) Engelm.

RAPD markers provide a powerful tool for the investigation of genetic variation in natural and domesticated populations. Recent studies of strain/cultivar identification have shown extensive RAPD divergence among, but little variation within, inbred species or cultivars. In contrast, little is known about the pattern and extent of RAPD variation in heterogeneous, outcrossing species. We describe the population genetic variation of RAPD markers in natural, diploid sources of dioecious buffalograss [Buchloë dactyloides (Nutt.) Engelm.]. Buffalograss is native to the semi-arid regions of the Great Plains of North America, where it is important for rangeland forage, soil conservation, and as turfgrass. Most sources of buffalograss germplasm are polyploid; diploid populations are previously known only from semi-arid Central Mexico. This is the first report of diploids from humid Gulf Coastal Texas. These two diploid sources represent divergent adaptive ecotypes. Seven 10-mer primers produced 98 polymorphic banding sites. Based on the presence/ absence of bands, a genetic distance matrix was calculated. The new Analysis of Molecular Variance (AMOVA) technique was used to apportion the variation among individuals within populations, among populations within adaptive regions, and among regions. There was considerable variation within each of the four populations, and every individual was genetically distinct. Even so, genetic divergence was found among local populations. Within-population variation was larger and among-population variation smaller in Mexico than in Texas. The largest observed genetic differences were those between the two regional ecotypes. These patterns of genetic variation were very different from those reported for inbred species and provide important baseline data for cultivar identification and continuing studies of the evolution of polyploid races in this species.

Nomenclatural Changes and New Taxa in Claytonia (Portulacaceae) in Western North America

Nomenclatural Changes and New Taxa in Claytonia (Portulacaceae) in Western North America

RECURRENT FORMATION AND POLYPHYLY OF NORDIC POLYPLOIDS IN DRABA (BRASSICACEAE)

Draba (Brassicaceae) is well known for its taxonomic complexity in arctic and alpine floras, and the polyploids in particular present vexing taxonomic problems. It has been suggested that polyploids in Draba may have formed recurrently from different populations of the parental species (polytopy), and it is also possible that a given taxonomic species may actually comprise several polyploid races, each originating from different progenitor species (polyphyly). To unravel the taxonomic complexity of polyploid Draba in the Nordic area, we investigated three of the most morphologically variable species and their possible progenitors using enzyme electrophoresis and restriction site analysis of chloroplast DNA (cpDNA) and nuclear ribosomal RNA genes (rDNA): D. norvegica (6x), D. lactea (6x), and D. corymbosa (16x). Electrophoretic analyses of progeny showed high levels of fixed heterozygosity in all three polyploids, demonstrating that all are genetic alloploids. Electrophoretic and rDNA data indicate that polytopic and/or polyphyletic origins have contributed to the complexity of these polyploids. However, a lack of cpDNA variation among the species limited the usefulness of this molecule for analysis of polyploid origins. The considerable electrophoretic variation observed in D. norvegica necessitates a minimum of three and probably 13 independent origins. Electrophoretic and rDNA data suggest that D. lactea and D. corymbosa are polyphyletic polyploids. Crossing data also support that D. corymbosa is polyphyletic. Given the widespread geographic distributions of these species and their possible progenitors, and that the populations analyzed represent only a small fraction of their geographic distributions, it is likely that these species have formed numerous times in different areas. As more molecular analyses of polyploids are completed, the data continue to suggest that multiple origins of polyploids are the rule rather than the exception.

Chromosome counts in Gaultheria and related genera

The chromosome numbers of 32 taxa in the ‘Gaultheria group’ of genera of the tribe Andromedeae in the Ericaceae have been counted; 15 of these have been counted for the first time. All taxa counted in Gaultheria and Leucothoë have a chromosome number based on x = 11. The chromosome numbers counted in Diplycosia are 2n = 36 and in Zenobia 2n = c.66. G. insana and G. procumbens show different polyploid races: G. insana tetraploids and hexaploids and G. procumbens diploids, tetraploids and octoploids.

CHROMOSOME NUMBERS IN CUPHEA (LYTHRACEAE): NEW COUNTS AND A SUMMARY

The American genus Cuphea with ca. 260 species is extremely diverse with respect to chromosome number. Counts are now available for 78 species and/or varieties, or 29% of the genus. Included in this study are first reports for 15 taxa from Brazil, Cuba, Dominican Republic, Mexico, and Venezuela. Twenty‐two different numbers are known for the genus, ranging from n = 6 to n = 54. The most common number in the primary center of species diversity in Brazil is n = 8, which is regarded as the base number of the genus. Two numbers are most common in the secondary center in Mexico, n = 10 and n = 12. Species with n = 14 or higher are considered to be of polyploid origin. Polyploids comprise 46% of the total species counted and appear in 9 of the 11 sections for which chromosome numbers have been reported. Aneuploid species comprise ca. 25% of the genus and are known from 7 of the 11 sections. The two subgenera are not characterized by different chromosome numbers or sequences of numbers. None of the 14 sections are circumscribed by a single chromosome number. Morphological and ecological variability in widespread, weedy species is correlated with differing chromosome numbers in some species whereas in others the chromosome number is stable. Summary of chromosome numbers by taxonomic section is presented. Section Euandra, centered in eastern Brazil, and the largest section of the genus, appears to be chromosomally most diverse. In section Trispermum, characterized by difficult, variable species with intermediate forms, two of the four species studied have polyploid races. Section Heterodon, endemic to Mexico and Central America and comprising most of the annual species of the genus, is best known chromosomally. Chromosome numbers have been counted for 25 of 28 species, and 12 different numbers are reported. The most advanced sections, Melvilla and Diploptychia, with numerous species occurring at higher altitudes, are characterized by high polyploids. Apomictic species occur in sect. Diploptycia. The cytoevolution of Cuphea is complex with frequent polyploid and aneuploid events apparently playing a significant role in speciation in both centers of diversity.

Chromosome Numbers in Cuphea (Lythraceae): New Counts and a Summary

The American genus Cuphea with ca. 260 species is extremely diverse with respect to chromosome number. Counts are now available for 78 species and/or varieties, or 29% of the genus. Included in this study are first reports for 15 taxa from Brazil, Cuba, Dominican Republic, Mexico, and Venezuela. Twenty-two different numbers are known for the genus, ranging from n = 6 to n = 54. The most common number in the primary center of species diversity in Brazil is n = 8, which is regarded as the base number of the genus. Two numbers are most common in the secondary center in Mexico, n = 10 and n = 12. Species with n = 14 or higher are considered to be of polyploid origin. Polyploids comprise 46% of the total species counted and appear in 9 of the 11 sections for which chromosome numbers have been reported. Aneuploid species comprise ca. 25% of the genus and are known from 7 of the 11 sections. The two subgenera are not characterized by different chromosome numbers or sequences of numbers. None of the 14 sections are circumscribed by a single chromosome number. Morphological and ecological variability in widespread, weedy species is correlated with differing chromosome numbers in some species whereas in others the chromosome number is stable. Summary of chromosome numbers by taxonomic section is presented. Section Euandra, centered in eastern Brazil, and the largest section of the genus, appears to be chromosomally most diverse. In section Trispermum, characterized by difficult, variable species with intermediate forms, two of the four species studied have polyploid races. Section Heterodon, endemic to Mexico and Central America and comprising most of the annual species of the genus, is best known chromosomally. Chromosome numbers have been counted for 25 of 28 species, and 12 different numbers are reported. The most advanced sections, Melvilla and Diploptychia, with numerous species occurring at higher altitudes, are characterized by high polyploids. Apomictic species occur in sect. Diploptycia. The cytoevolution of Cuphea is complex with frequent polyploid and aneuploid events apparently playing a significant role in speciation in both centers of diversity.

5S Nuclear Ribosomal Gene Variation in the Glycine tomentella Polyploid Complex (Leguminosae)

Glycine tomentella, a perennial wild relative of the soybean, is a species complex consisting of several reproductively isolated diploid (2n = 38, 40) and polyploid (2n = 78, 80) races. Variation in the repeat lengths of the nuclear multigene family encoding 5S ribosomal RNA was assayed for 20 diploid and 30 polyploid accessions from Australia, Papua New Guinea, Philippines, and Taiwan. These accessions represent all of the reproductively isolated and isozymically differ- entiated groups known to occur in the complex, including the recently described segregate, G. arenaria. Diploid G. tomentella accessions were found to have one of three size classes of 5S repeat, with good correlation between size class and isozyme group; G. arenaria possessed a repeat different in length from all G. tomentella accessions. Of the six classes of polyploids known from the complex, four had two major size classes of 5S repeats in each individual, three of them combining sizes found in the diploids. Two other polyploid isozyme races had only a single major repeat class. Hypothetical parents for hybrid origins of polyploids are suggested, and the consistency of these hypotheses with previously published evidence from isozyme and hybrid meiotic analysis is dis- cussed.

A cytological study ofPelargonium sect.Eumorpha (Geraniaceae)

The chromosome numbers of seven species ofPelargonium sect.Eumorpha have been determined from material of known wild origin, and karyotypic comparisons have been made. Within the section there is variation in basic chromosome number (x = 4, 8, 9, 11), variation in chromosome size, and two species have polyploid races. The three species with chromosome numbers based on x = 11 have the smallest chromosomes (1.0–1.5 µm); chromosomes are larger (1.0–3.0 µm) in the other species.P. elongatum has the lowest chromosome number in the genus (2n = 8).P. alchemilloides is exceptional in that it has four cytotypes, 2n = 16, 18, 34 and 36, and the form with 2n = 36 has large chromosomes (2.0–5.0 µm). Evidence from a synthesized hybrid suggests thatP. alchemilloides with 2n = 16 may be of polyploid origin. The three species based on x = 11 appear to be more closely related to species from other sections ofPelargonium that have the same basic chromosome number and small chromosome size, rather than to other species of sect.Eumorpha.

Seed proteins and systematics of Tripsacum

The genus Tripsacum (Gramineae) is distributed between the latitude 42°N and 24°S in the New World. It is divided into two sections. Section Tripsacum includes 11 species with T. dectyloides (L) L. extending across the range of the genus. Section Fasciculata includes five Meso-American species with T. lanceolatum Rupr. ex Fourn. extending into southern Arizona. The genus displays considerable diversity in seed proteins. Variation patterns are of limited use in distinguishing sections, but are species and habitat specific. Protein data are particularly useful in subspecific classification, and consistently distinguish diploid from polyploid races of T. zopilotense Hern. and Randolph and T. bravum Gray. The tetraploid ectotype of T. bravum deserves specific rank and the robust ecotype of T. dactyloides var. meridonate de Wet and Timothy deserves varietal rank.

Patterns of Morphological Variation in a Population of Mixed Species of Castilleja (Scrophulariaceae)

A large population of mixed species of Castilleja was sampled for morphological variability, with each of 100 collected plants being keyed to species, many on a best fit basis. Using taximetric methods, including cluster, discriminant, and principal components analyses, patterns of similarities were established and related to key-assigned species determinations. The complementary results of the analyses indicated that four taxonomic species formed two major groups: C. parviflora vs. C. elmeri, C. rhexifolia, and C. hispida. The morphological overlap within the second group suggests extensive hybridization and backcrossing characteristic of a hybrid swarm, with C. hispida and C. elmeri linked by the intermediate C. rhexifolia. In addition, three apparent hybrid specimens involving C. parviflora indicate that interspecific barriers are incomplete for this species. The taxonomic complexity of Castilleja has long been recognized and is generally attributed to hybridization and polyploidy, promoted by an allogamous breeding system and the foraging behavior of hummingbird and bumblebee pollinators. Based on reports by Heckard (1968) and Heckard and Chuang (1977), the majority of species are either polyploid at one or more levels or have polyploid races. A lack of meiotic irregularities suggests hybrid (allopolyploid) origins for most polyploid taxa (Heckard and Chuang 1977). Thus, as stated by Heckard (1964), hybridization contributes to complexity by forming new self-perpetuating variants and by obscuring discontinuity between parental species. Heckard (1968) reported that hybridization was also possible between taxa of different ploidy levels. Because Castilleja is a (hemi-)parasite, certain morphological traits, e.g., stem height and degree of branching (Holmgren 1971), may be influenced by its relationship with the host, thus adding to variation within taxa. However, Hansen (1979) observed that physiological variation induced by environmental stresses was moderated by parasitism (see also Heckard 1962). In any case the host-parasite relationship undoubtedly affects the pattern of variability in Castilleja. In this paper we present results and interpretations of a taximetric study (Anderson 1981) of a large and variable population of mixed Castilleja species. The several-thousand-plant population was located within an area of ca. 50 hectares on Slate Mountain, Hart's Pass, near Mazama, Washington (fig. 1). The habitat-type was a moist to wet subalpine meadow with rock outcrops and an east-southeast aspect. Associated plants represented a wide variety of families including Compositae, Ericaceae, Gramineae, Polygonaceae, Rosaceae, and Scrophulariaceae, all containing species reported by Heckard (1962) to be Castilleja hosts. We attempted to analyze the total range of variation among morphological characters available at time of flowering. The use of a large number of characters is in accordance with the argument of Sneath and Sokal (1973), i.e., the greater the number of characters, the better the resulting classification. The large number of characters tended to emphasize differences and resulted in low similarity values among plants. Still, distinct groupings did occur and these were in general agreement with species assignment based on key characters and species descriptions (Ownbey 1959; Holmgren 1974). However, variation was considerable, blurring species limits and suggesting hybridization, which had previously been recorded for all our key-assigned species (Ownbey 1959; Heckard and Chuang 1977) excepting C. parviflora.

A triploid race of the parthenogenetic Japanese weevil Blosyrus japonicus Sharp (Coleoptera: Curculionidae).

The chromosomes of the weevil, Blosyrus japonicus Sharp, have been investigated in Hokkaido. A triploid parthenogenetic race (3x=33) was found in Ishiyama-rikkyo, Sapporo. The species is now known to have four parthenogenetic races, 3x, 4x, 5x, 6x, as well as a diploid bisexual race. All these races have been found in Japan. This represents the highest number of different polyploid races observed in any parthenogenetic weevil.

A Revised Classification of Antennaria (Asteraceae: Inuleae) of the Eastern United States

Review of morphology, cytology, breeding behavior, and field observations has resulted in a revised classification of Antennaria as represented in the eastern United States. Four sexual diploid (n = 14) are recognized, namely A. plantaginifolia, A. neglecta, A. solitari'a, and A. virgznica. These diploids act as pillars that support a polyploid complex consisting of A. Parlinii s.l. with two subspecies including A. Parlinii subsp. fallax, comb. nov., and A. neodioica s.l. with subspecies, including A. neodioica subsp. canadensis, comb. nov., and A. neodioica subsp. petaloidea, comb. nov. Many of the polyploids are apomictic and that has affected taxonomic difficulties. Antennaria is a genus of dioecious, perennial, entire-leaved herbs that are distributed in temperate to arctic regions throughout the northern hemisphere with in the southern Andes of South America. The genus probably contains 20 to 30 sexual diploid as well as many heteroploid agamic complexes derived from them. Antennaria was split off as a separate genus from Gnaphalium L. by Gaertner (1791). Taxonomic problems in the genus began to be identified in the late 1890s when botanists, namely Greene, E. Nelson, Fernald, and Rydberg, began discovering and naming variant populations as species. Apomixis in the genus has led to the formation of many distinct polyploid races or clones, many of which were named as by these and other workers. A survey of the Gray Herbarium Card Index revealed that over 300 names have been proposed for North American Antennaria since 1897. Many were based on single collections from local populations that have probably never been recollected. Two opposing views are prevalent as to how Antennaria of the eastern United States should be classified. Fernald (1945, 1950) recognized the following 14 from the eastern United States: A. Brainerdii, A. campestris, A. canadensis, A. fallax, A. Farwellii, A. munda, A. neglecta, A. neodioica, A. Parlin ii, A. petaloidea, A. plantaginifolia, A. rupicola, A. solitaria, and A. virginica. By contrast, Cronquist (1945, 1952) recognized three fairly well marked species including the small-leaved A. neglecta, the large-leaved, polycephalous A. plantaginifolia, and the large-leaved, monocephalous A. solitaria. A survey of floras published after 1945 has revealed that opinion

ON THE LIFE CYCLE, CYTOLOGY AND TAXONOMY OF CLADOPHORA CALLICOMA FROM INDIA

The present investigation deals with the cultural observations on the morphology, reproduction, life cycle, cytology and taxonomy of a freshwater Cladophora, C. callicoma (L.) Kütz, from India. There is a regular isomorphic alternation of generations between quadriflagellate zoospore‐producing diploid plants and biflagellate isogamete‐producing haploid plants, coupled with heterothallism in the latter. Chromosome numbers of n = 12 and 2n = 24 were determined respectively for gametophytes and sporophytes. Twelve bivalents were counted in meiosis where chiasmata varied from one to three. In light of above observations, the affinity of C. callicoma with C. glomerata, to which the former has often been assigned, has been discussed. It is proposed that the C. glomerata complex, which consists of intraspecific polyploid races, might have two distinct lines of evolution with regard to life cycle; one with forms having higher ploidy levels and lacking an alternation of generations, and the other with forms having low ploidy levels and alternation of generations.

CYTOGEOGRAPHICAL STUDIES IN THE GENUS ARNICA (COMPOSITAE: SENECIONEAE). I

A total of 134 chromosome counts representing 21 taxa of the genus Arnica are presented. Counts are published here for the first time for A. lonchophylla (n = 38) and A. nevadensis (n = 38). Ten previously unreported counts representing 9 taxa are also presented. The basic chromosome number of the genus is x = 19. With respect to chromosome number, the genus exhibits maximum diversity in the Rocky Mountains of Colorado and Wyoming. Arnica angustifolia, A. cordifolia and A. mollis are recognized as mature polyploid complexes, containing several wide ranging polyploid races and only a few highly restricted or scattered diploid races. Within the genus in general, diploids tend to be restricted to unglaciated areas while polyploids are much wider ranging, particularly in unglaciated areas.

Polyploidy in insect evolution.

Of all living organisms insects are the group with the highest number of existing species. It is, of course, true that a fraction of the total number of insects has been cytologically studied. Polyploid forms are rare exceptions among them. Polyploidy in insects is always associated with the parthenogenetic mode of reproduction. The cytologically verified cases are described. As for the geographic distribution of polyploid insects, they have successfully colonized vast land areas. Their distributions are, in general, northern and montane. The polyploid races are in general far more widespread than their diploid bisexual ancestors. The possible models of origin of polyploid insects are covered as well as data on their gaenetic variability. There are apparent environmental correlations in the distribution of certain forms. Most polyploid insects have life cycles extending over two (or more) years. They are also in general flightless forms. Hypotheses on the relation between heterozygosity in polyploids as well as the consequences of mutations in polyploid lineages are also presented.

Origins of polyploids.

1. Polyploidy is a conspicuous feature of chromosomal evolution in higher plants. It is common in many genera, and numerous species are characterized by diploid and polyploid races. 2. Polyploid evolution is a process not an event. 3. Polyploid may involve somatic chromsome doubling or sexual functioning of cytologically non-reduced gametes. 4. Spontaneous chromosome doubling, either in the zygote to produce a polyploid is plant or in apical meristem to produce a polyploid chimera, is a rare event. 5. The common mode of polyploidy is through the formation and sexual functioning of cytologically non-reduced gametes. Increased in chromosome number can occur in the first or later hybrid generations. 6. Polyploid via cytologically non-reduced gametes is commonly a two step process. A diploid (2n) female gamete is fertilized by a haploid (n) male gamete to produce a triploid (3x), which in turn produces cytologically non-reduced triploid (3n) female gametes that are fertilized by haploid (n) gametes of the diploid parents and result in tetraploid (4x) offspring. 7. Fertilization of a rare diploid (2n) female gamete by an equally rare diploid (2n) male gamete to directly produce a tetraploid (4x) is extremely rare but does occur. 8. Polyploidy is successful only if the new polyploids are able to complete with their parents. Success depends on availability of suitable habitals, as well as the ability to produce successful offspring. 9. The most successful polyploids combine the diploid genomes of cytogenetically allied, but differently adapted taxa. 10. Fertility is restored in polyploids through cytological diploidization of the genomes or through gametophytic apomixis. 11. Reversible tetaploidy is part of polyploid evolution.

Comparative ecophysiology of the chromosome races in Viola adunca J.E. Smith.

Comparisons of net CO2 assimilation, dark respiration, leaf resistance, and leaf water potential were made between diploid and polyploid races of Viola adunca from the Cypress Hills, Alberta, Canada. The mean maximum net CO2 assimilation rate, at 20 C and 500 μE m-2 s-1 (phAR) was 26 mg CO2 g-1 h-1 (12 mg CO2 dm-2 h-1) for polyploids, and 23 mg CO2 g-1 h-1 (11 mg dm-2 h-1) for diploids. The difference is not statistically significant. Net CO2 assimilation rates at low (0° C) and high (40° C) temperatures were virtually the same for diploids and polyploids. There were no statistically significant differences between the chromosome races in light compensation or light saturation over the 0° to 40° C temperature range studied. Average dark respiration of the polyploid race at 20 C was 2.2 mg CO2 g-1 h-1 (1.0 mg CO2 dm-2 h-1) compared with 2.0 mg CO2 g-1 h-1 (0.95 mg CO2 dm-2 h-1) for the diploid race. The mean maximum leaf water potential of well watered plants was-7.9 bars for both ploidy levels. Minimum leaf resistance was ca. 3.6 s cm-1 for both ploidy levels. Maximum net CO2 assimilation rates in both ploidy levels occurred at-9 bars leaf water potential. Based upon the plant responses studied, there are no differences between chromosome races collected from the same general area, and the polyploids do not respond more favorably to extremes of temperature and water potential. Ploidy per se does not affect the response of Viola adunca to its environment in this particular case.

Genomic differentiation and polyploidy in Sowerbaea (Liliaceae)

Sowerbaea is an endemic Australian genus of five allopatric, vicarious species. A basic genome of three long subacrocentric chromosomes and one shorter acrocentric chromosome is constant in the genus, but within this overall genomic uniformity there are distinct karyotypic differences between species. There are also quantitative differences in nuclear DNA with a twofold difference between the species with the greatest and least amounts of DNA per genome. The quantitative DNA measurements have been combined with measurements of relative chromosome length to obtain estimates of differences in absolute chromosome sizes. Polyploidy is widespread in the genus, and diploid (n = 4), tetraploid (n = 8) and octoploid (n = 16) biotypes occur. Structural differences in the genomes were established at the diploid level, and have arisen during an early stage in differentiation of biotypes in the genus. The polyploid forms are mostly located in eastern Australia, and the establishment of polyploid races was probably a later phase of biotype differentiation associated with late Tertiary and Quaternary climatic changes. The tetraploids are structural autoploids, and have probably arisen within established biotypes. The octoploid form of S. juncea is probably an autoallo-octoploid, derived from tetraploid stocks which were ancestral to S. juncea and S. subtilis respectively.