Mapping diplosporous apomixis in tetraploid Tripsacum: one gene or several genes?

Abstract

Polyploids in Tripsacum, a wild relative of maize, reproduce through the diplosporous type of apomixis, an asexual mode of reproduction through seeds. Diplosporous apomixis involves both the failure of meiosis and the parthenogenetic development of the unreduced gametes, resulting in progenies that are exact genetic copies of the mother plant. Apomixis is believed to be controlled by one single dominant allele, responsible for the whole developmental process. Construction of a linkage map for the chromosome controlling diplosporous apomixis in Tripsacum was carried out in both tetraploid-apomictic and diploid-sexual Tripsacum species using maize restriction fragment length polymorphism (RFLP) probes. A high level of collinearity was observed between the Tripsacum chromosome carrying the control of apomixis and a duplicated segment in the maize genome. In the apomictic tetraploid, there was a strong restriction to recombination, as compared to the corresponding genomic segment in sexual plants and maize. This suggests that apomixis, although inherited as a single Mendelian allele, might really be controlled by a cluster of linked loci. The analysis also revealed the tetrasomic nature of the inheritance of the chromosomal segment controlling apomixis, which contradicts the usually accepted hypothesis of an allopolyploid origin of apomictic species. The implications of these data for the transfer of apomixis into cultivated crops are discussed, and a new approach to studying the genetics of apomixis, based on comparative mapping, is proposed.

Is supernumerary chromatin involved in gametophytic apomixis of polyploid plants?

“…Recent research documented that ploidy changes can quantitatively and qualitatively alter gene expression (Comai 2000;Wendel 2000). However, these hypotheses relating to alteration of, or interaction between, gene expression(s) do not involve any apomixis-linked genomic region and are by themselves difficult to reconcile with findings from mapping efforts in Brachiaria, Tripsacum, and Pennisetum-Cenchrus, Erigeron, (Pessino et al 1997;Grimanelli et al 1998a;Ozias-Akins et al 1998;Roche et al 1999;Noyes and Rieseberg 2000) in which large tracts of DNA have been linked to the trait.…”

“…These non-Mendelian transmissions may be tentatively explained by negative effects on meiosis or female gametophyte fitness, or by unknown phenomena of imprinting upon fertilization (Dobson and Tanouye 1998a). (4) The lack of recombination around the diplospory locus in Tripsacum (Grimanelli et al 1998a) and Erigeron annuus (Noyes and Rieseberg 2000) as well as the partially hemizygous ASGR in both P. squamulatum and C. ciliaris (Ozias-Akins et al 1998;Roche et al 1999) could be explained by a supernumerary nature of those genomic regions. (5) Many B-chromosomes seem to be involved in "anomalies" of meiotic or post-meiotic events in plants (Jones and Rees 1982).…”

“…Then, in some aspects we would be revisiting Ernst's 1918 hypothesis in which the author postulated that apomixis was a default pathway in interspecific hybrids induced by poor chromosome pairing at female meiosis. A few authors already acknowledge there may be no need of particular genes for the chronologically terminal element of apomixis that is the autonomous development of an unreduced egg (Mogie 1988;Nogler 1995;Grimanelli et al 1998a). This default scenario leads to the exciting prospect of inducing apomixis in certain genetic backgrounds by cytotoxic engineering targeted at the early meiotic stages of the megaspore mother cell.…”

“…Recent work on Tripsacum (Kindiger et al 1996;Grimanelli et al 1998a) seems to indicate that the genetic control of meiotic non-reduction resides on a single chromosome. In this species the involvement of an unknown number of genes within a non-recombining section of a chromosome is hypothesized (Savidan 2000).…”

Apomixis--The Asexual Revolution

“…In Tripsacurn (4), the gene or genes for apomixis map to a region densely populated with molecular markers (where polymorphisms abound or recoverv of recombinants is rare or nil), suggesting potential involvement of alien chromatin, pericentromeric heterochromatin, inversion heterozygosity, or complementary eametocidal loci.…”