Reproduction mode in the allopolyploid facultatively apomictic hawkweed Hieracium rubrum (Asteraceae, H. subgen. Pilosella)

摘要

Hieracium subgen. Pilosella (Hill) S.F. Gray has an extremely diverse breeding system. Besides clonal growth, both sexual reproduction and aposporous apomixis are present (Krahulcová et al. 2000; Bicknell et al. 2003). Although the sexuals are predominantly allogamous (Gadella 1984), autogamy is stimulated under the influence of foreign pollen (Krahulcová et al. 1999). Facultatively apomictic polyploids, acting as seed parents, can generate hybrids as well as an autonomously derived progeny via both somatic and haploid parthenogenesis (Skalińska 1971). Moreover, the apomicts are capable as pollen donors (Gadella 1987; Krahulcová and Krahulec 2000). The crosses involving the fusion of unreduced gametes, 2n+n or 2n+2n, respectively, give rise to “addition hybrids” (Skalińska 1976; Gadella 1988; Chapman and Bicknell 2000; Bicknell et al. 2003), i.e. BIII hybrids in the sense of Rutishauser (1967). In nature, the fertilization of unreduced egg cells by reduced sperm cells seems to be more common than vice versa, especially with facultative apomicts as seed parents (unpubl.).The initial formation of a meiotic (reduced) embryo sac in an ovule, usually suppressed by the rapid development of ameiotic (unreduced) female gametophyte, is characteristic of apospory (Koltunow 1993; Koltunow et al. 1998), although it is possible for both sexual and aposporous embryo sacs to coexist within one ovule. Hieracium subgen. Pilosella species produce endosperm autonomously, which is rather exceptional among aposporous apomicts, as most are pseudogamous, requiring pollination for endosperm production (Richards 1997). The dominant genetic control of aposporous apomixis has been demonstrated in several plant genera (Koltunow et al. 2000), including Hieracium subgen. Pilosella (Gadella 1987; Bicknell et al. 2000). The introduction of molecular developmental markers into sexual and apomictic plants of Hieracium subgen. Pilosella showed that sexual and apomictic pathways are closely interrelated, sharing regulatory programs (Tucker et al. 2003). In general, the degree of apomixis, i.e. the expression of residual sexuality, can be influenced by the dosage of an aposporous gene, as in the polyploid Ranunculus auricomus (Nogler 1984). It can also be influenced by the pollen donor or environmental factors, e.g. photoperiod and temperature (Asker and Jerling 1992; Koltunow 1993; Richards 1997). The environmental control of the timing of development of the sexual and aposporous embryo sacs within an ovule is likely to be critical for the mechanism of environmental control. Conditions of high temperature, low humidity and intense insolation are suggested to increase the frequency of embryos derived from sexual embryo sacs in facultatively aposporous Hieracium pilosella (Turesson 1972).Bicknell et al. (2003) quantified the proportions of particular progeny classes formed by two facultatively apomictic Hieracium species, H. aurantiacum and H. piloselloides, by screening seedling groups to determine their origins. The isolation of non-maternal individuals was based on the segregation and inheritance of introduced marker genes. The progeny class frequencies were estimated using a set of equations corresponding to particular pollination treatments. To study the reproduction in H. rubrum, we chose a different, less complicated method, which enabled an unambiguous detection of particular reproduction pathways operating in the polyploid facultatively apomictic mother plants. The reproductive origins, reflected in ploidy levels of the progeny, were sorted in this respect by flow cytometry. This approach was facilitated by an appropriate choice of ploidy for the pollen plants used in crosses. Another method proposed for detection of reproductive origins of progeny (Matzk et al. 2000), is based on flow-cytometric assessment of the embryo/endosperm ploidy ratio in seeds. To assess and quantify the progeny classes in H. rubrum, we performed the analysis of seeds as well. Th