Crosses between sexual and apomictic dandelions (Taraxacum). I. The inheritance of apomixis.

Some dandelions, Taraxacum, are diplosporous gametophytic apomicts. Crosses between closely related diploid sexuals and triploid apomicts were made to study the inheritance of apomixis. Seed-set was less than one-third of that in diploid x diploid crosses, probably because of the inviability of aneuploid pollen or zygotes. Almost 90% of the viable offspring were diploid and the result of selfing, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor effect on self-pollen, causing a breakdown of the sporophytic self-incompatibility system. A similar phenomenon has been reported before in wide crosses. Of the 26 allozyme-confirmed hybrids, four were diploids, 15 were triploids and seven were tetraploids. Diploid hybrids were significantly less frequent than triploid hybrids, suggesting either low fitness of haploid pollen or more numerous formation of diploid pollen. Emasculation and bagging of flowers indicated apomictic seed-set in none of the diploid, in one-third of the triploid and in all of the tetraploid hybrids. All apomictic hybrids showed partial seed-set, but additional cross-pollination did not increase seed-set. Cytological analysis of the F2 progeny confirmed that partial apomixis was caused by semisterility and not by residual sexuality (facultative apomixis). The difference in segregation for apomixis between triploid and tetraploid hybrids may be because the triploids originated from partially reduced diploid pollen grains, whereas the tetraploids originated from unreduced triploid pollen grains.

Crosses between sexual and apomictic dandelions (Taraxacum). II. The breakdown of apomixis.

Some dandelions are diplosporous gametophytic apomicts. In order to study the inheritance and breakdown of apomixis, crosses were made between diploid sexuals and triploid apomicts. To investigate their breeding system, four nonapomictic diploid and 10 nonapomictic triploid hybrids were pollinated with diploids and the progenies were analysed. Seed fertility was significantly reduced in two diploid hybrids. Nine triploid hybrids were fertile and could be classified into three types, with respect to the composition of their progenies. Type A produced n+n hybrids. Type B produced either a mixture of n + n and 2n + n hybrids, or a mixture of pseudogamous 2n + 0 apomicts and 2n + n hybrids. Type C produced exclusively 2n + n hybrids. Inheritance of a microsatellite marker strongly suggested that 2n egg cells in type C plants were produced by a first division restitution mechanism. As in apomicts, microsporogenesis in type C plants was reductional. This suggests that type C plants are diplosporous plants that lack parthenogenesis. Such plants are very rare in other apomictic plant species. It is concluded that 'elements of apomixis', diplospory and parthenogenesis, can be uncoupled. This is inconsistent with the single-locus model for apomixis in Taraxacum as suggested by Mogie (1992). Instead, our results suggest that several loci are involved in the genetic control of apomixis in Taraxacum.