Nuclear-cytoplasmic male-sterility in diploid dandelions.

Male-sterility was found in diploid dandelions from two widely separated populations from France, and its inheritance was analysed by crossing a diploid male-sterile dandelion to diploid sexuals and triploid apomicts. Nuclear genetic variation, found in full-sib families, segregated for male-fertility, partial male-sterility, and full male-sterility, and also segregated for small-sized versus normally sized pollen. The crossing results are best explained by a cytoplasmic male-sterility factor in combination with two dominant restorer genes. Involvement of the cytoplasmic male-sterility factor was further investigated by chloroplast haplotyping. Male-sterility was exclusively associated with a rare chloroplast haplotype (designated 16b). This haplotype was found in seven male-sterile plants and one (apparently restored) male-fertile individual but does not occur in 110 co-existing male-fertile plants and not in several hundreds of individuals previously haplotyped. Apomicts with cytoplasmic male sterility were generated in some test crosses. This raises the question as to whether the male sterility found in natural dandelion apomicts, is of cytoplasmic or of nuclear genetic nature. As many breeding systems in Taraxacum are involved in shaping population structure, it will be difficult to predict the evolutionary consequences of nuclear-cytoplasmic male-sterility for this species complex.

A genetic linkage map of the diplosporous chromosomal region in Taraxacum officinale (common dandelion; Asteraceae).

In this study, we mapped the diplosporous chromosomal region in Taraxacum officinale, by using amplified fragment length polymorphism technology (AFLP) in 73 plants from a segregating population. Taraxacum serves as a model system to investigate the genetics, ecology, and evolution of apomixis. The genus includes sexual diploid as well as apomictic polyploid, mostly triploid, plants. Apomictic Taraxacum is diplosporous, parthenogenetic, and has autonomous endosperm formation. Previous studies have indicated that these three apomixis elements are controlled by more than one locus in Taraxacum and that diplospory inherits as a dominant, monogenic trait ( Ddd; DIP). A bulked segregant analysis provided 34 AFLP markers that were linked to DIP and were, together with two microsatellite markers, used for mapping the trait. The map length was 18.6 cM and markers were found on both sides of DIP, corresponding to 5.9 and 12.7 cM, respectively. None of the markers completely co-segregated with DIP. Eight markers were selected for PCR-based marker development, of which two were successfully converted. In contrast to all other mapping studies of apomeiosis to date, our results showed no evidence for suppression of recombination around the DIP locus in Taraxacum. No obvious evidence for sequence divergence between the DIP and non- DIP homologous loci was found, and no hemizygosity at the DIP locus was detected. These results may indicate that apomixis is relatively recent in Taraxacum.

Genetic structure of a population sample of apomictic dandelions.

In Northern Europe, dandelion populations consist solely of triploid or higher polyploid apomicts. Without a regular sexual cycle or lateral gene transmission, a clonal structure is expected for Taraxacum apomicts, although this was not found by compatibility analysis. In this study, we investigate whether this observation could be suported by performing independent tests based on data from hypervariable microsatellite markers as well as more conservative data based on allozymes and matrilinear cpDNA markers. In addition, population genetic methods were used to test departure from panmictic expectations, which is expected for clonal populations. Results indicated that many data sets, again, did not agree with expectations from clonal evolution because only small groups of genotypes exhibit no marker incompatibility. Population genetic analysis revealed that virtually all genotypes, but not individuals, agreed with random segregation and genotypic equilibria. Exceptions were genotypes with rare allozyme alleles or nearly identical microsatellite genotypes. Consequently, a population sample of apomictic dandelions essentially harbours genotypes that resulted from segregation and/or recombination and only a few genotypes that may have differentiated by somatic mutations.