High-resolution physical mapping reveals that the apospory-specific genomic region (ASGR) in Cenchrus ciliaris is located on a heterochromatic and hemizygous region of a single chromosome

Abstract

An apomictic mode of reproduction known as apospory is displayed by most buffelgrass (Cenchrus ciliaris) genotypes, but rare sexual individuals have been identified. Previously, intraspecific crosses between sexual and aposporous genotypes allowed linkage to be discovered between the aposporous mode of reproduction and nine molecular markers that had been isolated from an aposporous relative, Pennisetum squamulatum. This region was described as the apospory-specific genomic region (ASGR). We now show an ideogram of the chromosome complement for aposporous tetraploid buffelgrass accession B-12-9 including the ASGR-carrier chromosome. The ASGR-carrier chromosome has a region of hemizygosity, as determined by in situ hybridization of BAC clones and unique morphological characteristics when compared with other chromosomes in the genome. In spite of its unique morphology, the ASGR-carrier chromosome could be identified as one of the chromosomes of a meiosis I quadrivalent. A similar partially hemizygous segment was also detected in the ASGR-carrier chromosome of the aposporous buffelgrass genotype, Higgins, but not in the sexual accession B-2S. Two non-recombining BACs linked to apospory were physically mapped on a highly condensed chromatin region of the short arm of B-12-9, and the distance between the BACs was estimated to be approximately 11 Mbp, a distance similar to what previously has been shown in P. squamulatum. The short arm of the ASGR-carrier chromosome was highly condensed at pachytene and extended only 1.7-2.7 fold that of mitotic chromosomes. Low recombination in the ASGR may partially be due to its localization in heterochromatin.

Molecular cytogenetics and DNA sequence analysis of an apomixis-linked BAC in Paspalum simplex reveal a non pericentromere location and partial microcolinearity with rice

“…Several hypotheses have been formulated to explain the causes of the block of recombination described in apomictic systems (Grimanelli et al 1998;Ozias-Akins et al 1998;Labombarda et al 2002;Goel et al 2003); among these the physical location of the ACL in heterochromatindense pericentromeric regions that are known to be recombinationally suppressed appears worthy of investigation. Recent results obtained for C. ciliaris seem to confirm this hypothesis since the apomixis locus of this species is located near the centromere in a heterochromatin-rich region (Akiyama et al 2005). Several characteristics associated with heterochromatin have been observed at the same locus of P. squamulatum, although it is not located close to the centromere (Akiyama et al 2004).…”

“…In a recent paper, Goel et al (2003) reported for the first time the FISH-aided localization of the apomixis locus on metaphase chromosomes and they claimed that its vicinity to the centromere in Cenchrus ciliaris and to centromeric repeats in P. squamulatum could cause the repression of recombination at the same locus in these species. Further FISH analyses on pachytene chromosome preparations of the same species are in favour of these assumptions (Akiyama et al 2004;Akiyama et al 2005;Ozias-Akins et al 2003). FISH using pachytene chromosomes as targets offers superior spatial resolution of adjacent probes compared to metaphase chromosomes and display clearly the dense heterochromatic regions on the chromosomes and so allow the assignment of a probe position in relation to the structural organization of its surrounding chromosome context (de Jong 2003).…”

The polyploid nature of Cenchrus ciliaris L. (Poaceae) has been overlooked: new insights for the conservation and invasion biology of this species – a review

“…This asexual reproduction provides the advantage of rapid multiplication allowing the development of a significant aboveground yield and consequently the formation of an extremely dense ground cover by C. ciliaris (Ozias-Akins et al 2003;Akiyama et al 2005;Miles 2007;Ozias-Akins and Van Dik 2007;Singh et al 2007). Asexual reproduction may have benefits in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity allowing adaptation to a changing environment.…”

“…Cytogenetic investigations of tetraploid C. ciliaris have demonstrated that the ASGR is located near the centromere in a genomic region that is hemizygous and heterochromatic (condensed part of chromatin genetically inactive) (Goel et al 2003(Goel et al , 2006Ozias-Akins et al 2003;Akiyama et al 2005). The ASGR-carrier chromosome has a unique morphological characteristic when compared with other chromosomes in the genome (Akiyama et al 2005) and is~20 Mb larger than its presumed homologous chromosomes (Goel et al 2003;Akiyama et al 2005). Goel et al (2003) suggest that chromosome structure, proximity to centromere and hemizygosity (the state of having only one copy of a gene in the diploid cell instead of the usual two copies) may all play a role in the low recombination rates in the ASGR.…”

“…High similarity among individuals belonging to the same offspring was found, indicating that the measured attributes were highly heritable. This no doubt results from the apomictic reproductive mode of C. ciliaris that leads to the formation of seeds genetically identical to the mother plant (Ozias-Akins et al 2003;Akiyama et al 2005). As stated above, apomixis is a heritable trait (Grimanelli et al 2001;Jessup et al 2002;Curtis and Grossniklaus 2007;Ozias-Akins and Van Dik 2007;Yadav et al 2012), but is rarely obligatory, and such is the case for C. ciliaris.…”