Karyotypes of the polyploid parthenogenetic species Saga pedo from four localities in France and the Republic of Macedonia were constructed and compared. All these karyotypes consist of 70 chromosomes, which is more than twice that in other species of the genus. The chromosomes differ from each other, making the matching of homologues difficult. Karyotypes of French specimens are similar, except for differences in the heterochromatin. Compared to that of the Macedonian specimens those from French specimens differ by the shortening of a single chromosome. The difficulty experienced in identifying tetrads and even pairs of chromosomes indicates that either many chromosome rearrangements have occurred since the polyploidisation event(s) or that the addition of quite different genomes is the cause. On the other hand, that the karyotypes are similar indicates a common origin of both the Macedonian and French populations. Thus, most chromosome changes preceded the separation from their common ancestor. Both the DNA content and chromosome analyses suggest that the S. pedo karyotype is pentaploid and not tetraploid as previously proposed. This odd ploidy number rules out the hypothesis that it could only have originated by endoreduplication. It is more likely that it originated by the association of five copies of the 14,X haploid karyotype, which exists in the gametes of the closely related species, S. campbelli and S. rammei (male / female 2n = 27, X / 28, XX). Fertilization of a parthenogenetic 56, XXXX female by a 14, X spermatozoa could have resulted in the last increase in ploidy.
Comparative analyses of interspecific data in evolutionary biology usually require specific methods to remove the effects of phylogenetic inertia. When phylogenetic inertia is not considered, the Canarian Pimelia species show a positive, and almost significant (Prob. = 0.066) correlation between nuclear genome size and body size. However, after controlling for phylogenetic inertia there was a negative and significant correlation (Prob. = 0.007 to 0.017, depending on the DNA fraction considered). Such a change in the relationship after controlling for phylogenetic inertia is rarely reported. Moreover, the relationship usually reported is positive and thought be a consequence of species having a similar number of cells at the same stage of development. The aim of the present study is to report a case of a negative correlation, but not to explain the causal mechanism involved in genome size variations or propose a formal hypothesis on the specific links between DNA content and body size. However, a common explanation of the change in the relationship, i.e., positive to negative, is suggested. Moreover, the data available on the highly repetitive, non-coding satellite DNA allows us to analyse the specific pattern exhibited by this fraction.
A population of a hybrid between Tragopogon porrifolius and T. pratensis (T. ×mirabilis), which occurs in SW part of the town of Roudnice nad Labem, N part of Central Bohemia, was analysed with respect to its morphology, fertility, life history, ploidy level and DNA content. Both parental species vary relatively little morphologically; they are biennials (monocarpic perennials) and diploids. T. pratensis is a native species in the Czech Republic, T. porrifolius was cultivated there in the past. The hybrid plants are extremely morphologically variable, with variation ranges of some characters overlapping those of the parental species (e.g. ligules are often longer than involucral bracts, peduncles are often lanate). Only diploids were found within the hybrid population; however, they have substantially lower DNA content than both parents (18% lower than T. pratensis, 42% lower than T. porrifolius). The plants of the Roudnice hybrid population are polycarpic perennials in contrast to the monocarpic perennial (mostly biennial) parents. The distribution is described in detail; it shows that the hybrid plants are spreading and at present even occur outside the town. The long-persisting population of fertile diploid hybrid plants in Roudnice nad Labem is an alternative evolutionary pathway to that of the allotetraploid Tragopogon species known from North America.
Intraspecific variation in genome size makes it possible to study ongoing processes of genome size evolution. Although there are over 200 papers on intraspecific variation in genome size, there is still limited understanding of this phenomenon, especially as many of these papers are based on weak methodology and therefore report biased or false evidence of the extent of intraspecific variation. In this paper the recent progress in understanding the spatio-temporal dynamics of intraspecific variation in genome size caused by the gradual accumulation of mutations is reviewed. The results of the case studies on Microseris douglasii, Zea mays, Silene latifolia, Hordeum spontaneum and Lolium hybrids, and in particular that on Festuca pallens, are discussed. The variation in genome size that occurs within species is caused mainly by differences in the content of repetitive DNA, in particular it is a consequence of the dynamics of transposable elements. Variation may be induced and maintained polytopically.We assume that it is probably more frequent in groups of young radiating species. Even in the initial stages, the variation in genome size generated within a population seems to be restricted by selection, which is also important in stabilizing genome size within species. The long-term persistence of the variation within a population and its further accumulation may be enhanced by gametes with different genome sizes, produced by the segregation of unequally sized homeologous chromosomes. Over large geographical scales and across contrasting environmental gradients, the distribution of genome sizes within species may be influenced by the nucleotype effect, with smaller genomes being more successful at higher latitudes and altitudes and under stressful conditions. However, the small differences in genome size within species seem generally to be of minor importance relative to other components of plant fitness that may be selectively favourable under particular environmental or habitat conditions. The processes generating variation in genome size may be associated with phenotypic variation. While the shift in the genome size of a population through selection enables adaptive evolution of genome size in a newly arising species, the spatio-temporal variation in genome size within an ancestral species allows for a rapid multiple genome size divergence of related species through random drift in genome size (founder effect, bottleneck effect) during range fragmentation, hybridization and/or polyploidization.