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

Abstract

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´diploid crosses, probably because of the inviability of aneuploid pollen or zygotes. Almost 90% of the viable o spring were diploid and the result of sel®ng, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor e ect 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-con®rmed hybrids, four were diploids, 15 were triploids and seven were tetraploids. Diploid hybrids were signi®cantly less frequent than triploid hybrids, suggesting either low ®tness of haploid pollen or more numerous formation of diploid pollen. Emasculation and bagging of¯owers 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 F 2 progeny con®rmed that partial apomixis was caused by semisterility and not by residual sexuality (facultative apomixis). The di erence 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.

The role of tetraploids in the sexual–asexual cycle in dandelions (Taraxacum)

“…Inviable aneuploid pollen from triploid apomicts causes a mentor effect for self-pollen of diploid sexuals, leading to a breakdown of the SI system (Morita et al, 1990b). Nonetheless, Tas and Van Dijk (1999) could confirm some diploid and polyploid offspring as true hybrids in sexual diploid-apomictic triploid crosses by allozyme markers. Reciprocal apomictic triploid-sexual diploid crosses produce generally only apomictic triploid offspring, but occasionally also low numbers of diploid offspring have been reported (Richards, 1970a;Mü ller, 1972).…”

“…From each seed head, 20-50 seeds were germinated and the ploidy level of the seedlings was determined. In 1993, this was carried out by mitotic chromosome number counts (see Tas and Van Dijk 1999), in the other 2 years by flow cytometry.…”

“…Samples taken in 1993 and 1995 indicated that sexual diploid and asexual triploid Taraxacum section Ruderalia plants grew intermingled at this site. In 1999 more than 2100 plants were sampled and analysed by flow cytometry (Ploidy Analyser; Partec, Muenster, Germany, see Tas and Van Dijk, 1999) to determine the cytotype frequencies of established plants.…”

“…Tas and Van Dijk (1999) found that triploid hybrids from experimental sexual-apomict crosses were not always apomicts, but segregated for apomixis and the degree of apomixis. Apomictic reproduction in Taraxacum involves three developmental processes or elements: diplospory (first meiotic division restitution), parthenogenetic embryo growth and autonomous endosperm development.…”

“…This suggests that genes can readily flow from the triploid apomicts into the diploid sexual gene pool. However, allozyme markers indicated that nearly all diploid offspring were produced by induced selfing and not caused by hybridization (Morita et al, 1990a;Tas and Van Dijk, 1999). Inviable aneuploid pollen from triploid apomicts causes a mentor effect for self-pollen of diploid sexuals, leading to a breakdown of the SI system (Morita et al, 1990b).…”