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

Abstract

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...

The Genetic Control of Apomixis: Asexual Seed Formation

“…The presence of varied apomictic mechanisms and the phylogenetic positioning of apomictic species throughout many angiosperm families together imply that apomixis has evolved independently multiple times (Carman 1997;Van Dijk and Vijverberg 2005). Genetic analyses using apomicts as pollen donors in crosses with sexual individuals as maternal parents have shown apomixis to be inherited as a dominant trait.…”

“…Studies involving genetic analyses searching for rare recombinants in suppressed regions and gamma deletion mutagenesis have revealed that in some species, the developmental components of apomixis (meiotic avoidance, parthenogenesis, and fertilization-independent endosperm development) are controlled by independent loci. For instance, in Taraxacum and Erigeron species, two independent loci have been identified that control diplospory and parthenogenesis (Van Dijk et al 1999;Noyes and Rieseberg 2000). Similarly, apospory and parthenogenesis are controlled by two independent loci in Hypericum, Poa, Hieracium, and Cenchrus species (Albertini et al 2001;Catanach et al 2006;Schallau et al 2010;Conner et al 2013).…”

Seed development and inheritance studies in apomictic maize-Tripsacum hybrids reveal barriers for the transfer of apomixis into sexual crops

“…Variation in the reproductive behavior within a single individual is common in both natural and artificial apomicts (Koltunow et al, 2000, Leblanc, 1995, Nogler, 1984, Noyes, 2005, Richards, 1996, Van Dijk et al, 1999. In order to assess the nature and the extent of variation in reproductive behavior in 38C, an apomictic clone, we collected flow cytometrical, cytogenetical and molecular data within progenies obtained after pollination using 2x maize progenitors.…”

Inheritance of apomeiosis (diplospory) in fleabanes (Erigeron, Asteraceae)

“…In aposporous Poa pratensis (Poaceae), for example, apomeiotic individuals (identified cytologi-cally) constituted 5% of F1s (Albertini et al, 2001). In diplosporous Taraxacum officinale (Asteraceae), six of 19 F1s (32%) were identified as apomeiotic, producing at least some 2n þ n and no 2n þ 0 offspring progeny when test-crossed (van Dijk et al, 1999). In Erigeron (Asteraceae), diplosporous plants constituted 23% (30/130) of an F1 mapping population generated by crossing apomictic E. annuus with sexual E. strigosus.…”