Non-Mendelian transmission of an apospory-specific genomic region in a reciprocal cross between sexual pearl millet ( Pennisetum glaucum ) and an apomictic F1 ( P. glaucum × P. squamulatum )

Abstract

We recently showed that aposporous apomixis, a form of gametophytic apomixis, is controlled by a single apospory-specific genomic region (ASGR) in both Pennisetum squamulatum and Cenchrus ciliaris. We present evidence that in a reciprocal cross between sexual pearl millet (P. glaucum) and an apomictic F1 (P. glaucum × P. squamulatum) the ASGR is not transmitted at the same rate. When pearl millet was used as the female parent and the apomictic genotype as the pollen donor, the ASGR was transmitted at a rate of 0.41 in a progeny of 57 plants, indicating a slight transmission ratio distortion. However, in a population of 52 rare sexual progenies characterized among a large progeny of a quasi-obligate apomict (an F1 hybrid of P. glaucum × P. squamulatum), the transmission rate of ASGR was only 0.12. This strong segregation distortion may have occurred at four different levels: (1) female meiosis, (2) during female gametophyte maturation, (3) upon fertilization with differential survival of embryos being a consequence of differential gene expression controlled by parent-of-origin specific effects (imprinting) and (4) at a later developmental stage of the embryo through an embryo/endosperm genetic incompatibility system.

Crossability and Characterization of Interspecific Hybrids between Sexual Pennisetum glaucum (Pearl Millet) and a New Cytotype (2n=56) of Apomictic P. squamulatum

“…Segregation for certain characters, such as node colour and hairiness, violet stigma colour, tillering behaviour, and other traits observed in the hybrids suggests heterozygous nature of P. squamulatum MA, since Tetra 1 was homozygous being maintained by recurrent self-pollination. Some of these morphological characters were similar to hybrids reported between tetraploid pearl millet and 2nϭ54 cytotypes of P. squamulatum (Dujardin and Hanna 1983, 1985, Marchais and Tostain 1997, Roche et al 2001. These reports too suggested the heterozygous nature of P. squamulatum.…”

“…Marchais and Tostain (1997) produced fertile hybrids using tetraploid pearl millet and explained the successful hybridization events on the basis of endosperm balance number theory. Similar explanations were provided by Roche et al (2001) in interspecific crosses involving Pennisetum species.…”

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

“…Whether maternal repression of apomixis mechanistically relies on similar bases is unknown, but note that, in most cases investigated including Tripsacum, "apomixis DNA segments" consist of non-recombining chromosomal blocks. Finally, reports in Pennisetum indicate similar strong segregation distortion for the apospory-specific genomic region (AGSR) when inherited through sexual female gametes (Roche et al, 2001). Interestingly, the two DNA segments associated with apomixis, and cytogenetically best characterized to date, contain rearrangements of high repetitive sequences derived from transposable elements (Akiyama et al, 2004, Calderini et al, 2006.…”

Understanding Apomixis: Recent Advances and Remaining Conundrums

“…Similarly, the inheritance of diplospory in Eastern gamagrass (Tripsacum dactyloides) is reported to be simple and dominant (Leblanc et al, 1995). There is evidence of segregation ratio distortion in some of these systems, often because the dominant factor(s) associated with apomixis also appears to confer gamete lethality, restricting its transfer to some gamete genotypes (Nogler, 1984b;Grimanelli et al, 1998a;Roche et al, 2001a;Jessup et al, 2002).…”

“…An apparent interspecific hybrid origin also is a common feature among apomicts, and the combination of polyploidy and hybridity is believed to have resulted in allopolyploidy in many gametophytic apomicts (Ellerstrom and Zagorcheva, 1977;Carman, 1997Carman, , 2001Roche et al, 2001a). The action of tetrasomic inheritance in many systems, however, also indicates the presence of autopolyploidy, or possibly segmental allopolyploidy, in these plants (Pessino et al, 1999).…”