Hybridization in natural mixed populations of sexual diploid and apomictic triploid dandelions (Taraxacum sect. Taraxacum): Why are the diploid sexuals not forced out?

摘要

In agamic complexes, the 'minority cytotype exclusion principle' can be modified to the 'sexual cytotype exclusion principle': Apomicts produce polyploid progeny of maternal genotype without the need for pollination, but still produce pollen. Outcrossing sexuals thus suffer from unsuccessful hybridizations - they produce reduced amounts of progeny. This causes a decrease in the relative frequency of sexuals and later leads to their exclusion from the population. However, in the agamic complex Taraxacum sect. Taraxacum, diploid sexuals coexist with triploid apomicts in stabilized mixed populations. This paper deals with hybridizations in natural populations, the possibilities of new triploid apomicts originating and investigates why the 'sexual cytotype exclusion principle' does not work in Taraxacum sect. Taraxacum. The progeny of diploids from natural populations screened by the flow-cytometric seed screening method consisted only of diploids. Different simulations of natural pollinations were carried experimentally. When the flowers of a diploid received different mixtures of pollen of surrounding diploids and triploids, pollination was successful and only diploid progeny was produced, probably due to the preference of high-quality compatible pollen of diploids by diploid maternal plants. Therefore, the 'sexual cytotype exclusion principle' cannot work in Taraxacum sect. Taraxacum, and diploids and triploids can coexist in common populations. Polyploid progeny was produced by a diploid plant surrounded only by triploid apomicts to a certain distance. This suggests that new polyploid apomicts can be produced only in places where isolated diploids can occur among triploids. Newly formed apomicts further spread their progeny to both areas of apomicts and areas of mixed populations.