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

Antony M Chettoor1, Scott A Givan2, Rex A Cole3, Clayton T Coker1, Erica Unger-Wallace4, Zuzana Vejlupkova2, Erik Vollbrecht4, John E Fowler3 and Matthew MS Evans1*

Author Affiliations

1 Department of Plant Biology, Carnegie Institution for Science, Stanford 94305, CA, USA

2 Informatics Research Core Facility, University of Missouri, Columbia 65211, MO, USA

3 Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331, OR, USA

4 Department of Genetics, Development and Cell Biology, Iowa State University, Ames 50011, IA, USA

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Genome Biology 2014, 15:414  doi:10.1186/s13059-014-0414-2

Published: 31 July 2014

Abstract

Background

Plant gametophytes play central roles in sexual reproduction. A hallmark of the plant life cycle is that gene expression is required in the haploid gametophytes. Consequently, many mutant phenotypes are expressed in this phase.

Results

We perform a quantitative RNA-seq analysis of embryo sacs, comparator ovules with the embryo sacs removed, mature pollen, and seedlings to assist the identification of gametophyte functions in maize. Expression levels were determined for annotated genes in both gametophytes, and novel transcripts were identified from de novo assembly of RNA-seq reads. Transposon-related transcripts are present in high levels in both gametophytes, suggesting a connection between gamete production and transposon expression in maize not previously identified in any female gametophytes. Two classes of small signaling proteins and several transcription factor gene families are enriched in gametophyte transcriptomes. Expression patterns of maize genes with duplicates in subgenome 1 and subgenome 2 indicate that pollen-expressed genes in subgenome 2 are retained at a higher rate than subgenome 2 genes with other expression patterns. Analysis of available insertion mutant collections shows a statistically significant deficit in insertions in gametophyte-expressed genes.

Conclusions

This analysis, the first RNA-seq study to compare both gametophytes in a monocot, identifies maize gametophyte functions, gametophyte expression of transposon-related sequences, and unannotated, novel transcripts. Reduced recovery of mutations in gametophyte-expressed genes is supporting evidence for their function in the gametophytes. Expression patterns of extant, duplicated maize genes reveals that selective pressures based on male gametophytic function have likely had a disproportionate effect on plant genomes.