Characterization of Apomictic BC₇ and BC₈ Pearl Millet: Meiotic Chromosome Behavior and Construction of an ASGR‐carrier Chromosome‐specific Library

Abstract

Apospory in Pennisetum squamulatum Fresen is inherited as a single‐locus dominant trait. Molecular analysis has identified the apospory‐specific genomic region (ASGR), a large segment of one chromosome that is necessary and sufficient to confer apomixis. To transfer apospory from P. squamulatum to pearl millet [P. glaucum (L.) R. Br.], advanced backcrosses were generated. Here we report the characterization of apomictic BC7 and BC8 generations. A BC7–derived genotype has two P. squamulatum chromosomes, while BC8–derived apomictic lines inherited only the ASGR chromosome that confers apospory to pearl millet. Morphologically, no significant differences were observed between backcross generations for plant height, leaf length, leaf width, and first internode and inflorescence lengths. However, the BC7 and BC8 lines differed significantly with regard to pollen viability, flowering time, and seed set. Pollen viability increased to 81% in the BC8 lines compared with 37% in BC7. The frequency of aposporous embryo sacs was lower in BC8 (79%) lines compared to BC7 (93%). Meiotic behavior of chromosomes during metaphase I and anaphase I showed that BC8 typically had a single univalent, identified as the ASGR chromosome by FISH (fluorescence in situ hybridization) and GISH (genomic in situ hybridization). Utilizing its unique behavior during meiosis the ASGR chromosome was microdissected and a chromosome‐specific library was constructed and sequenced. Analysis of sequences identified both novel and previously known ASGR‐linked sequences.

A parthenogenesis gene of apomict origin elicits embryo formation from unfertilized eggs in a sexual plant

“…CcASGR-BBM-like1 (EU559278.1) is transcribed and 99.7% identical to the PsASGR-BBML across the ORF, whereas CcASGR-BBM-like2 (EU559279.1), also transcribed, contains two nonsense mutations. Expression of ASGR-BBML has been observed by RT-PCR in apomictic P. squamulatum (PI 319196), C. ciliaris (B12-9), and Pennisetum glaucum backcross (BC) 7 (06-63) and BC 8 (06-A-58) lines (13) in unfertilized ovaries starting 1 d before anthesis and assayed up to 2 d after anthesis, at which time ovaries without endosperm development begin to senesce; in pollinated ovaries at day of anthesis through early seed development; in anthers 1 d before anthesis; and in roots. PsASGR-BBML is also expressed in embryogenic callus derived from apomictic P. glaucum BC 8 (06-A-58).…”

Apomixis in plant reproduction: a novel perspective on an old dilemma

“…In the case of Tripsacum , the current evidence suggests the presence of barriers in the transfer of apomixis to maize (Leblanc et al 2009). The greatest progress has been made with the introgression of apomixis into pearl millet from Pennisetum , where a single alien chromosome persists in the most advanced apomictic lines (Singh et al 2010; Zeng et al 2011). In the sexual model plant Arabidopsis , for each element of apomixis identified by mutation so far, the penetrance was shown to be generally low and the related candidate genes were never found to be associated with any feature of apomixis in natural apomicts.…”

In Silico and Fluorescence In Situ Hybridization Mapping Reveals Collinearity between the Pennisetum squamulatum Apomixis Carrier-Chromosome and Chromosome 2 of Sorghum and Foxtail Millet

“…Apomictic P . glaucum backcross 8 (BC 8 ) (06-A-58) derived seedlings carrying the ASGR-carrier chromosome [ 12 ] were used for FISH and tissue collection for RNA extraction. Backcross 06-A-58 is a facultative apomict and therefore produces progeny derived through both modes of reproduction, apomictic and sexual.…”

“…glaucum , was identified by FISH to carry one alien chromosome, the ASGR-carrier chromosome from P . squamulatum [ 12 ]. Forty-nine contigs, generated from the assembly of 454 sequences derived from dissected apomictic ovules, were mapped to the ASGR-carrier chromosome via SCAR, CAPS or SSCP markers [ 13 ].…”