The oomycetous fungus Phytophthora infestans (Mont.) de Bary, which causes late blight disease in potatoes, is heterothallic with two known mating types, AI and A2. From 1845 until 1980 only A1 mating type isolates were found in Europe. In 1980, the A2 mating type appeared permitting sexual reproduction. Here we show that virulence properties and DNA fingerprint patterns of isolates collected in the Netherlands before and after the appearance of A2 mating type isolates are different. Before 1980, eight different races were found in which virulence factors 1, 2, 3, 4 and 10 were most common. After 1980, new virulence factors (i.e. 5, 6, 7, 8 and 11) showed up and the diversity for virulence increased tremendously: 73 different races were detected among 253 isolates analyzed. DNA fingerprint analyses of isolates collected before 1980 revealed that, for at least two decades, only one RG-57 fingerprint genotype was present in Europe. Among 179 isolates collected after 1980 134 distinct RG-57 fingerprint genotypes were identified. The dramatic increase in genetic diversity strongly suggests that the P. infestans population in the Netherlands is now propagating sexually. The change from asexual to sexual reproduction, and the resulting increased adaptability and ability to survive outside the host, may interfere drastically with the regular disease control methods.

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.

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 ospring were diploid and the result of sel®ng, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor eect 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 dierence 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.

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 signi®cantly reduced in two diploid hybrids. Nine triploid hybrids were fertile and could be classi®ed 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 ®rst 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.Keywords: 2n-gametes, apomixis, diplospory, parthenogenesis, pseudogamy, Taraxacum. IntroductionIn apomictic plants seeds are produced asexually (Nogler, 1984; Asker & Jerling, 1992; Mogie, 1992; Koltunow, 1993). Apomictic seed development diers in at least three elements from sexual seed development: (i) avoidance of meiotic reduction; (ii) avoidance of fertilization; and (iii) parthenogenetic embryo development. This results in the production of 2n + 0 maternal ospring, instead of n + n sexual ospring (using the terminology of Harlan & DeWet, 1975). In most apomicts, the endosperm is formed sexually, after fertilization of the central cell of the embryo sac (pseudogamy). In autonomous apomicts, such as dandelions, Taraxacum, however, the endosperm develops autonomously. In this paper we report on the breakdown of apomixis in Taraxacum into its elements.Apomixis is a complex developmental trait and the prevailing opinion has for long been that the elements of apomixis are controlled by independent genes. In order to understand the genetics of apomixis, these elements should be analysed separately in crosses (Asker, 1980; Nogler, 1984). However, the extreme rarity of apomictic recombinants (either producing exclusively 2n + n hybrids or producing exclusively n + 0 polyhaploids) has raised doubts about the independent genetic control of elements of apomixis (Asker & Jerling, 1992). In their monograph on apomixis in plants, Asker & Jerling (1992) cite only three cases of plants producing exclusively 2n + n hybrids: Parthenium argentatum (Powers, 1945), Potentilla argentea (Asker, 1970) and Ranunculus auricomus (Nogler, 1984). Furt...

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 signi®cantly reduced in two diploid hybrids. Nine triploid hybrids were fertile and could be classi®ed 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 ®rst 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.Keywords: 2n-gametes, apomixis, diplospory, parthenogenesis, pseudogamy, Taraxacum. IntroductionIn apomictic plants seeds are produced asexually (Nogler, 1984; Asker & Jerling, 1992; Mogie, 1992; Koltunow, 1993). Apomictic seed development di ers in at least three elements from sexual seed development: (i) avoidance of meiotic reduction; (ii) avoidance of fertilization; and (iii) parthenogenetic embryo development. This results in the production of 2n + 0 maternal o spring, instead of n + n sexual o spring (using the terminology of Harlan & DeWet, 1975). In most apomicts, the endosperm is formed sexually, after fertilization of the central cell of the embryo sac (pseudogamy). In autonomous apomicts, such as dandelions, Taraxacum, however, the endosperm develops autonomously. In this paper we report on the breakdown of apomixis in Taraxacum into its elements.Apomixis is a complex developmental trait and the prevailing opinion has for long been that the elements of apomixis are controlled by independent genes. In order to understand the genetics of apomixis, these elements should be analysed separately in crosses (Asker, 1980; Nogler, 1984). However, the extreme rarity of apomictic recombinants (either producing exclusively 2n + n hybrids or producing exclusively n + 0 polyhaploids) has raised doubts about the independent genetic control of elements of apomixis (Asker & Jerling, 1992). In their monograph on apomixis in plants, Asker & Jerling (1992) cite only three cases of plants producing exclusively 2n + n hybrids: Parthenium argentatum (Powers, 1945), Potentilla argentea (Asker, 1970) and Ranunculus auricomus (Nogler, 1984). F...