Discovery of novel transcripts and gametophytic functions via RNA-seq analysis of maize gametophytic transcriptomes

This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by BioMed Central Ltd. The published article can be found at: http://genomebiology.com/.BACKGROUND:\ud Plant gametophytes play central roles in sexual reproduction. A hallmark of the plant life\ud cycle is that gene expression is required in the haploid gametophytes. Consequently, many\ud mutant phenotypes are expressed in this phase.\ud RESULTS:\ud We perform a quantitative RNA-seq analysis of embryo sacs, comparator ovules with the\ud embryo sacs removed, mature pollen, and seedlings to assist the identification of\ud gametophyte functions in maize. Expression levels were determined for annotated genes in\ud both gametophytes, and novel transcripts were identified from de novo assembly of RNA-seq\ud reads. Transposon-related transcripts are present in high levels in both gametophytes,\ud suggesting a connection between gamete production and transposon expression in maize not\ud previously identified in any female gametophytes. Two classes of small signaling proteins\ud and several transcription factor gene families are enriched in gametophyte transcriptomes.\ud Expression patterns of maize genes with duplicates in subgenome 1 and subgenome 2\ud indicate that pollen-expressed genes in subgenome 2 are retained at a higher rate than\ud subgenome 2 genes with other expression patterns. Analysis of available insertion mutant\ud collections shows a statistically significant deficit in insertions in gametophyte-expressed\ud genes.\ud CONCLUSIONS:\ud This analysis, the first RNA-seq study to compare both gametophytes in a monocot, identifies\ud maize gametophyte functions, gametophyte expression of transposon-related sequences, and\ud unannotated, novel transcripts. Reduced recovery of mutations in gametophyte-expressed\ud genes is supporting evidence for their function in the gametophytes. Expression patterns of\ud extant, duplicated maize genes reveals that selective pressures based on male gametophytic\ud function have likely had a disproportionate effect on plant genomes