Quantification of progeny classes in two facultatively apomictic accessions of Hieracium

Abstract

Hieracium is an established model system for studying the cytological and genetic basis of gametophytic apomixis. In common with most known apomicts, the formation of 'maternal seed' is not exclusive in Hieracium, as apomixis operates in conjunction with a low level of sexuality. When this occurs the form of apomixis is described as 'facultative'. The formation of maternal seed in these plants is characterised by the avoidance of meiosis followed by the parthenogenetic development of an unreduced egg cell. In some ovules, however, meiosis does proceed, and sometimes the fertilisation of an egg cell presages embryogenesis. As a result, this mechanism of facultative apomixis leads to the formation of several different types of progeny, each representing a unique combination of meiosis/apomeiosis and fertilisation/parthenogenesis. Furthermore, fertilisation may involve either self or non-self pollen, leading to the recognition of six progeny classes from each individual plant. To facilitate an understanding of these processes we have developed a method for identifying individuals from different progeny classes based on the inheritance of introduced heterologous marker genes. This technique permits the screening of many thousands of seedlings at germination, and the consequent isolation of individuals associated with rare classes. Progeny profiles were determined for two apomictic accessions of Hieracium. Both were found to develop approximately 2.5% of their seed from meiotically derived eggs under the experimental conditions used and to have a rate of hybridity of approximately 2%. Evidence was also found for the action of a self-incompatibility mechanism operating in these plants despite the autonomous nature of apomixis in Hieracinum. As a demonstration of the utility of this approach, a study was conducted of polyembryony in one accession. The results indicate that there was a 7 fold greater likelihood that a meiotically derived seedling would arise in a polyembryonic seed than in a single-embryo seed. This indicates that facultative apomixis in Hieracium not only results from the simultaneous occurrence of sexual and asexual seed formation in the same capitulum as previously demonstrated, but most often as parallel processes within the same ovule.

Genetic diversity and reproductive biology in ecotypes of the facultative apomict Hypericum perforatum L.

“…For example, the aposporic initial often appeared evident and active even at the time of MMC differentiation and, subsequently, the vast majority of embryo sacs most likely had an aposporic origin because they were centrally located within the ovule. Thus, the sexual process is usually terminated soon after the aposporic initial cell differentiates in H. perforatum, as has been observed in most apomictic Hieracium (Koltunow et al, 1998(Koltunow et al, , 2000Bicknell et al, 2003). Our observations are consistent with the hypothesis that the aposporic initial might directly influence the sexual product degradation at this early stage (Tucker et al, 2003).…”

Understanding Apomixis: Recent Advances and Remaining Conundrums

“…Dioecy is known in a number of apomicts, such as Antennaria (O'Connell and Eckert, 1999), Cortaderia (Philipson, 1978), and Coprosma (Heenan et al, 2002). Similarly, self-incompatibility, a known characteristic of the sexual biotypes of Hieracium subgenus Pilosella (Gadella, 1984(Gadella, , 1991Krahulcovà et al, 1999), was demonstrated recently in the apomictic species H. aurantiacum and H. piloselloides (Bicknell et al, 2003).…”

“…This allows the inheritance of apomixis to be studied using intraspecific hybridization, in contrast to many other apomictic plant systems. Methods for the micropropagation (Bicknell, 1994b), anther culture (Bicknell and Borst, 1996), genetic transformation (Bicknell and Borst, 1994), and progeny class estimation (Bicknell et al, 2003) of Hieracium have been developed to further facilitate its use in the molecular study of apomixis. The plants also are very amenable to vegetative propagation, which is of particular value when genetic or molecular manipulations affect flower form and/or seed set.…”

Use of the SSLP-based method for detection of rare apomictic events in a sexual AtSERK1 transgenic Arabidopsis population

“…This requested a reliable screening system that can differentiate between a rare apomictic and the sexual mode of reproduction in the progeny of the transgenic plants. So far, diverse methods for apomictic screening have been reported including flow cytometry analysis (Matzk et al 2001), a transgenic approach (Bicknell et al 2003), auxin induction (Matzk 1991a, b) and morphological analysis (Koltunow 1993). These methods, however, are either time consuming or they are not suitable to detect the sporophytic mode of apomixis.…”

“…Until now, apomicts were identified through morphological scoring, chromosome counting or progeny cross-methods, which were tedious and very time consuming. In Hieracium, a method based on the inheritance of positive and negative selectable transgenic markers have been described (Bicknell et al 2003). This method, based on the survival of different progeny types when placed on selective media, is however limited to plant species that can be transformed genetically.…”