Karyotype diversification and evolution in diploid and polyploid South American Hypochaeris (Asteraceae) inferred from rDNA localization and genetic fingerprint data

BACKGROUND AND AIMS

Changes in chromosome structure and number play an important role in plant evolution. A system well-suited to studying different modes of chromosome evolution is the genus Hypochaeris (Asteraceae) with its centre of species' diversity in South America. All South American species uniformly have a chromosome base number of x = 4 combined with variation in rDNA number and distribution, and a high frequency of polyploidy. The aim of this paper is to assess directions and mechanisms of karyotype evolution in South American species by interpreting both newly obtained and previous data concerning rDNA localization in a phylogenetic context.

METHODS

Eleven Hypochaeris species from 18 populations were studied using fluorescence in situ hybridization (FISH) with 35S and 5S rDNA probes. A phylogenetic framework was established from neighbour-net analysis of amplified fragment length polymorphism (AFLP) fingerprint data.

KEY RESULTS

A single 5S rDNA locus is invariably found on the short arm of chromosome 2. Using 35S rDNA loci, based on number (one or two) and localization (interstitial on the long arm of chromosome 2, but sometimes lacking, and terminal or interstitial on the short arm of chromosome 3, only very rarely lacking), seven karyotype groups can be distinguished; five of these include polyploids. Karyotype groups with more than one species do not form monophyletic groups.

CONCLUSIONS

Early evolution of Hypochaeris in South America was characterized by considerable karyotype differentiation resulting from independent derivations from an ancestral karyotype. There was marked diversification with respect to the position and evolution of the 35S rDNA locus on chromosome 3, probably involving inversions and/or transpositions, and on chromosome 2 (rarely 3) concerning inactivation and loss. Among these different karyotype assemblages, the apargioides group and its derivatives constitute by far the majority of species.

Evolutionary DNA variation in the genus hypochaeris

The genome size and the base composition (GC%) of eight Hypochaeris species were determined by flow cytometry in order to establish the pattern of nuclear DNA variation within the genus. The species analysed showed an almost fivefold range of variation from 1.68 pg in H. cretensis to 8.10 pg in H. uniflora. This variation in DNA content is greater between taxonomic sections of Hypochaeris species than within a section. There was no correlation between 2C DNA content and GC% indicating that neither the GC fraction nor the AT fraction were preferentially associated with variation in genome size. Because there is little heterochromatin, these results show that it is interspersed repeated sequences that are most probably implicated in this variation. From phylogenetic analysis, it is likely that genome size has evolved by loss of DNA content in some lineages and by gain in one lineage from an ancestral genome which was probably similar to genomes of intermediate size in Hypochaeris.

Genome size, fluorochrome banding, and karyotype evolution in some Hypochoeris species

Four South American and two European species of Hypochoeris (Asteraceae) were studied using fluorochrome banding, and genome size was determined by flow cytometry, in order to obtain information about microevolution in this genus and about its primary origin. Fluorochrome banding patterns showed GC-rich repeated sequences, particularly around the nucleolar organizer regions. Few differences appeared among the South American species. Nevertheless, determination of nuclear DNA content and base composition revealed significant differences among these species. The phylogenetic position of Hypochoeris robertia, which has the smallest DNA content, is discussed with regard to chromosome evolution in this genus.