Identification and characterization of apospory candidate genes in Pennisetum and Cenchrus

Abstract

Apomixis, asexual seed reproduction by avoidance of meiosis and fertilization of the egg cell, holds great potential for agriculture as a means to fix hybrid vigor since offspring generated through apomixis are genetic clones of the maternal plant. Apospory is a form of apomixis where the embryo develops from an unreduced egg that is derived from the aposporous embryo sac. In both Pennisetum squamulatum and Cenchrus ciliaris, apospory inherits as a dominant trait under genetic control of a single locus and transmits with an apospory-specific genomic region (ASGR). In order to understand the molecular mechanism regulating apospory, two major studies have been conducted. Aposporous initial specification is a critical event for the occurrence of apospory. To elucidate the mechanism controlling this process, two transcriptomes, derived from ovules of an apomictic donor parent and its apomictic backcross derivative at the stage of apospory initiation, were sequenced using 454-FLX technology. Analysis of the two transcriptomes allowed identification of the ASGR-carrier chromosome linked transcripts. Moreover, one of these alien expressed genes was assigned to the ASGR by screening a limited number of apomictic and sexual F1s. Identification of potential ASGR-linked candidate genes will provide significant insight into the regulation of apospory initiation. The results suggest that the strategy of comparative sequencing of donor parent and backcross transcriptomes is an efficient approach to identify alien expressed genes in a recurrent parent background. Previous functional analysis of BABY BOOM (BBM) genes from other species suggests that BBM plays a role in embryo development. To investigate the function of BABY BOOM-like genes identified from the ASGR compared with related genes not associated with the ASGR, ASGR-BBM-like and non-ASGR-BBM-like (N-ASGR-BBM-like) genes isolated from Cenchrus ciliaris were subjected to sequence analysis and expression characterization. Semi-quantitative RT-PCR with aposporous ovaries from different stages indicated that ASGR-BBM-like genes started transcription before pollination and the transcription was up-regulated upon pollination. In contrast, there was no expression of N-ASGR-BBM-like genes until one day after pollination. Moreover, the transcription level of N-ASGR-BBM-like was much lower than that of ASGR-BBM-like at this time point. Temporal and quantitative differences in transcript level between the two suggest that ASGR-BBM-like plays a role in parthenogenesis in buffelgrass.