Monogenic inheritance of apomixis in two Hieracium species with distinct developmental mechanisms

Abstract

Apomixis, the asexual formation of seed, has been known in angiosperms for more than a century yet the genetic mechanisms that control this trait remain poorly understood. Most members of the genus Hieracium are apomicts, forming predominantly asexual seed. Some purely sexual forms, however, also exist. In this paper we present a study of the inheritance of apomixis using two apomictic species of Hieracium which utilize very different forms of megagametogenesis. In both cases the progeny inherited apomixis as a monogenic, dominant trait that could be transferred by both haploid and diploid male gametes. In common with observations from other systems, no diploid apomictic progeny were recovered from these crosses. This appears to have been caused by selection against the survival of diploid zygotes, rather than against the mediation of haploid gametes as has been noted in other systems. Crosses between the two apomicts showed that the dominant determinants in the two forms examined were closely linked, possibly allelic. The significance of these data is discussed with respect to current theories on the associative link between gametophytic apomixis and polyploidy.

Levels of apomictic and amphimictic seed formation in a natural population ofCoprosma robusta(Rubiaceae) in Riccarton Bush, Christchurch, New Zealand

“…It has been suggested that, for many species, only diploid or polyploid gametes may carry the alleles for apomixis and that haploid gametes are selected against or have developmental abnormalities (Nogler 1984;Naumova et al 2001). In some cases, however, diploid apomicts have been recorded from the field (Jankun & Kovanda 1988;Naumova et al 1999Naumova et al , 2001) and haploid transfer of apomixis has been noted (Bicknell et al 2000), indicating that this effect may be associated with the genomic organisation of individual taxa, rather than a feature of apomixis per se. Given the close association of apomixis and polyploidy, it is perhaps most likely that in Coprosma the functional diploid chromosome number of 2n = 44 actually represents a tetraploid complement with a basic number of x = 11.…”

The Conserved Chimeric Transcript UPGRADE2 Is Associated with Unreduced Pollen Formation and Is Exclusively Found in Apomictic Boechera Species

“…apomeiosis). Furthermore, modifier genes (Bicknell et al, 2000), genetic background, and/or environmental conditions (Nogler, 1984) could explain variation in the level of apomictic trait expression ( Figs. 2A and 8).…”

Sexual and Apomictic Reproduction in Hieracium subgenus Pilosella Are Closely Interrelated Developmental Pathways

“…The coexistence of apomixis and sexual reproduction in an apomictic plant and the polyploid nature of most apomicts complicate genetic analyses. Nevertheless, it has been demonstrated that different modes of apomixis are conferred by a few Mendelian loci in which apomixis usually is dominant over sexual reproduction (do Valle and Savidan, 1996;Barcaccia et al, 1998;Tas and Van Dijk, 1999;Bicknell et al, 2000). Models proposed to explain the manifestation of apomixis suggest that it may be a novel pathway distinct from sexual reproduction (Roche et al, 1999) or an aberrant form of sexual reproduction generated by hybridization, mutation, or epigenetic change (Peacock, 1992;Carman, 1997;Grossniklaus, 2001).…”

“…This is not the case for composite Hieracium species, which display a generation time from seed to seed of 4 months and are suited to the molecular analysis of apomixis (Koltunow et al, 1995;Bicknell et al, 2000). Apomixis is dominant in Hieracium , and a number of developmentally characterized lines that exhibit differences in the frequency and timing of apomictic initiation in addition to variable modes of embryo sac formation have been genotyped (Koltunow et al, 1998Bicknell et al, 2000).…”