Ovule Transcriptomes of Weakly Aposporous and Non-aposporous F2 Sorghum Sibs

Ovule transcriptomes of two F₂ Sorghum bicolor sibs, one producing (14% of ovules) and one not producing aposporous embryo sacs (AES), were compared.  Megasporogenesis and sexual embryo sac formation occurred earlier during ovule development in the AES-forming sib, and this occurred whether AES were present or not.  This observation is consistent with previous results, based on 569 diverse sorghum genotypes, that suggested that sub-threshold influences of a weak apomixis mechanism in weakly aposporous sorghum accelerates germline development regardless of whether it occurs aposporously or sexually.  Ovules at the megasporocyte, meiocyte and late embryo sac formation stages were micro-excised from pistils of both sibs (five reps, 3410 pistils total), and total RNA was extracted, amplified and hybridized to CombiMatrix CustomArrays (synthesized from 12,000 inflorescence-associated sorghum Unigene EST).  About 80% of probes produced signals 2-fold greater than the SDEV of the background, and 231 of these were differentially expressed between aposporous and non-aposporous sibs at one or more stages of ovule development (P < 0.05, FDR corrected).  To identify functional relationships of these differentially expressed ESTs, gene sequence data for each EST were BLASTed against the TAIR10 protein database, and best fit Arabidopsis genes were identified (E value = 1.0 x 10^-30).  AmiGO gene ontology (GO) enrichment analyses were then performed based on TAIR annotations.  The differentially expressed genes produced 53 enriched GO categories including response to stress, response to stimulus, several ribosome and nucleolus-related categories, and redox-related categories, and these were consistent across low to high BLASTX E-value cutoffs.