CHARACTERIZING THE STIGMA TRANSCRIPTOME OF LEPTOSIPHON JEPSONII

Abstract

The majority of angiosperm species are hermaphroditic, containing both male and female reproductive organs on the same plant. This introduces the possibility of reproducing through self-fertilization (selfing) in flowering plants. The capacity for selfing has resulted in the evolution of a wide variety of mating systems, many of which promote outcrossing by deterring or completely preventing self-pollination. Notable among these is the mating system of Leptosiphon jepsonii, an annual plant from California that is in the family Polemoniaceae. Leptosiphon jepsonii exhibits a unique mixed mating system–utilizing both selfing and outcrossing at different times of maturation. During the first 24-48 hours of anthesis, L. jepsonii promotes outcrossing by expressing homomorphic, sporophytic self-incompatibility (SSI) –a molecular mechanism that rejects self-pollen or pollen originating from any plant that shares an allele at the S-locus (a term used to define any genetic locus that controls self-incompatibility). The reason the mating system of L. jepsonii is unique, however, is because this SSI is transient; after the first 24-48 hours of anthesis L. jepsonii becomes self-compatible. While many angiosperm species exhibit various kinds of mixed-mating systems, and others display genetic SSI, it appears that the combination of the two traits in a transient nature is uncommon. Consequently, identifying the processes involved in this mating system could aid in addressing ii multiple questions regarding mating system evolution, including whether mixed mating systems can be an evolutionarily stable state. To identify the processes active in the stigma of L. jepsonii, I performed a transcriptomic analysis of RNA-seq data collected from the leaf, petal, and stigma of three L. jepsonii populations. A global transcriptome was generated from all the collected samples; then differential expression analysis based on tissue type was used to identify potential genes that were upregulated in the stigma. Functional annotation analysis was used to find potential processes associated with unigenes that were upregulated in the stigma. Based on Gene Ontology analysis, cation transport, cell wall organization, enzyme inhibition, and glycosylation were the most prominent annotation vocabularies found among unigenes upregulated in the stigma. Within these broader categories, pectinesterase activity, peroxidase activity, UDP-glycosylation, and MtN3 family nodulin activity were most abundant among stigma-specific unigenes, and their expression patterns were compared with other angiosperm species with dry stigmas, as well as species with SSI. The most noteworthy finding is a combination of unigenes that resemble the S-locus glycoproteins (SLGs) and S-locus receptor kinases (SRKs) integral to the functioning of SSI in Brassicaceae. Four contigs representing three different unigenes had sequence similarity to Brassicaceae SLGs, and were expressed highly in the stigma while showing effectively no expression in the petal and leaf. In addition, one unigene with sequence similarity to Brassicaceae SRKs was expressed in flower tissue–the petal and stigma–but not the leaves. While these findings on their own require further validation, the candidate processes and components identified in this analysis provide a foundation for understanding how the L. jepsonii stigma, and its transient SSI, function.