Abstract
Dioecy is a rare sexual state in plants where male and female reproductive organs are produced by genetically distinct individuals. This means each plant is either male or female and cannot reproduce without the reciprocal sex’s gametes. Kiwifruit is a dioecious fruit crop that has high commercial relevance to the New Zealand economy. Sex in kiwifruit is controlled by two sex-determining genes encoded within a small non-recombining sex-determining region (SDR) on chromosome 25. One of these genes, a fasciclin-like arabinogalactan protein (FLA) Friendly Boy (FrBy) is required to produce fertile pollen. Female kiwifruit, which lack FrBy, produce infertile pollen that lacks the pectocellulosic intine cell layer, and anthers which undergo delayed tapetal programmed cell death (PCD).
To understand the genetic changes that underpin this abortive process, and to determine the function of FrBy, a protocol was developed to capture transcriptomic data from both a single anther and the isolated pollen from it, which was named Parallel Anther & Pollen RNA-seq (PAPR). Using PAPR, a comprehensive de novo transcriptome assembly was generated which was used to explore changes in expression across key developmental stages in anther and pollen development. In male kiwifruit anthers, novel insights into the role of the endoplasmic reticulum and the unfolded protein response (UPR) in pollen development were revealed. Further, it was shown that metabolic re-engineering occurs in late-stage pollen and anther development in male kiwifruit. PAPR also showed a female-specific upregulation of cell wall degradation enzymes which may be responsible for the intineless phenotype in female pollen. Based on the results from PAPR and previous experiments, it is proposed that FrBy is expressed in microspores after meiosis, and acts as a Ca2+ capacitor to release cations that cross-link to pectic polysaccharides in the intine, maintaining microspore integrity. Based on sequence analyses across plant lineages, it is further proposed that FrBy is descended from a clade-specific family of FLAs which evolved early in land plants to regulate spore development. Tissue-specific expression data in eudicots, Amborellales, and mosses obtained from open-access data repositories supported this hypothesis. Finally, it was shown that the SDR contains evidence of a recent intrachromosomal invasion of autosomal sequence, possibly highlighting a mechanism through which genes can be localised to an existing SDR and become sex-linked. These results have helped improve understanding of how FrBy contributes to male fertility in kiwifruit.