Abstract
BackgroundPlant 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.ResultsWe 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.ConclusionsThis 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-014-0414-2) contains supplementary material, which is available to authorized users.
Highlights
IntroductionA hallmark of the plant life cycle is that gene expression is required in the haploid gametophytes
Plant gametophytes play central roles in sexual reproduction
This study provides the first evidence for transposon expression in the male and female gametophytes of a plant with a large, complex genome containing many active transposon classes
Summary
A hallmark of the plant life cycle is that gene expression is required in the haploid gametophytes. For mutations affecting male and/or female gametophytes, the mutant allele (and the alleles of loci linked to it) is found at a reduced frequency in progeny when the defective gamete is involved (that is, male gametophyte mutants are recovered poorly when heterozygotes are crossed as males). This characteristic reduced transmission prevents, or makes very difficult, the generation of mutant homozygotes. Subgenome 2 is characterized by lower levels of gene expression and higher rates of gene loss than subgenome 1 [11]
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