The genetic structure of populations of sexual and asexual Taraxacum (dandelions)

Abstract

The genetic structure, as assessed by isozymes, is described for three populations of outbreeding sexuals, three populations of obligate agamosperms, and six accessions of inbreeding sexual Taraxacum. Fifteen loci in 10 isozyme systems were identified, and isozyme bands were previously shown to be allelic in sexual x sexual and were confirmed as allelic in sexual x agamosperm crosses. Sexual x agamosperm crosses gave rise to both diploid sexual and tetraploid agamospermous offspring. Eight loci were found to be monomorphic, or almost so, in outbreeding sexuals and the agamosperms. Six loci were polymorphic in sexual outbreeders and two of the agamosperm populations; the third agamosperm population was polymorphic at only four loci. For outbreeding sexuals, genotype frequencies were consistent with Hardy-Weinberg equilibria. For the agamosperm populations, 14 out of 16 polymorphic loci were invariably heterozygous. In outbreeding sexual populations nearly all individuals had different genotypes. For two agamospermous populations, all individuals had the same genotype. For the third, three genotypes occurred. For the agamosperms all offspring studied showed the maternal genotype. For four of the inbreeding sexual accessions, all of the original siblings showed the same genotype, and their offspring only had maternal genotypes. For the fifth, one locus was polymorphic, but only homozygotes occurred in either generation. The sixth inbreeding accession, which may be partially outbred, was heterozygous at two loci in one individual, and heterozygous at one locus in another. These loci segregated in the offspring. With respect to various measures of genetic variation, sexual outbreeders and agamosperms were similar for the proportion of loci polymorphic, but this was lower in the inbreeders. The number of alleles per locus were similar for the sexual outbreeders and agamosperms, but this was higher in the inbreeders. Twice as many loci per individual were heterozygous in the agamosperms as in the sexual outbreeders, but the proportion heterozygous in the inbreeders was very low. Genotypic diversity was very high in the sexual outbreeders, and very low in the agamosperms. It was also very low within accessions, but high between accessions for the inbreeders.

Hybridization between European and Asian dandelions (Taraxacum section Ruderalia and section Mongolica)

“…This report also showed that in two cases at least one facultative agamospermous plant occurred in progenies but all other progeny were sexuals. More recently, Muller (1972), Jenniskens (19846) and Hughes & Richards (1988) using Ruderalia plants, have reported further investigations. In Muller (1972), 260 progeny plants of 2x-3x crosses were sexual diploids and only nine progeny plants were triploid facultative agamosperms.…”

“…Recently Hughes and Richards (1988) concluded from electrophoretic analysis that diploid progeny from crosses beween a diploid microspecies of section Ruderalia and the triploid T. pseudohamatum were indeed hybrids. T. pseudohamatum Dahlst.…”

“…In Jenniskens (19846) only one of the 38 plants was triploid, 37 plants were found to be diploids (2« = 16 or 17). In Hughes & Richards (1988) 36 plants were diploids and 3 were tetraploids.…”

“…A few were facultative agamospermous eutriploids or nearly triploid aneuploids (Richards, ]970a-c;Muller, 1972;Jenniskens, 19846) or tetraploids (Hughes & Richards, 1988).…”

Genetic structure of a population sample of apomictic dandelions

“…A population sample may comprise, randomly sampled apomictic dandelions (eg Lyman and Ellstrand, 1984;Menken et al, 1995;Van der Hulst et al, 2000). Alternatively, a population sample may correspond to a set of apomicts, selected on the basis of certain morphological features that are diagnostic for microspecies (eg Hughes and Richards, 1988;. As our aim was to analyse genetic variation in a stand of apomicts and to infer whether apomicts have clonal relationships, we applied the first type of sampling.…”