Abstract
BackgroundCarica papaya is a trioecious plant species with a genetic sex-determination system defined by sex chromosomes. Under unfavorable environmental conditions male and hermaphrodite exhibit sex-reversal. Previous genomic research revealed few candidate genes for sex differentiation in this species. Nevertheless, more analysis is still needed to identify the mechanism responsible for sex flower organ development in papaya.ResultsThe aim of this study was to identify differentially expressed genes among male, female and hermaphrodite flowers in papaya during early (pre-meiosis) and later (post-meiosis) stages of flower development. RNA-seq was used to evaluate the expression of differentially expressed genes and RT-qPCR was used to verify the results. Putative functions of these genes were analyzed based on their homology with orthologs in other plant species and their expression patterns. We identified a Male Sterility 1 gene (CpMS1) highly up-regulated in male and hermaphrodite flower buds compared to female flower buds, which expresses in small male flower buds (3–8 mm), and that might be playing an important role in male flower organ development due to its homology to MS1 genes previously identified in other plants. This is the first study in which the sex-biased expression of genes related to tapetum development in the anther developmental pathway is being reported in papaya. Besides important transcription factors related to flower organ development and flowering time regulation, we identified differential expression of genes that are known to participate in ABA, ROS and auxin signaling pathways (ABA-8-hydroxylases, AIL5, UPBEAT 1, VAN3-binding protein).ConclusionsCpMS1 was expressed in papaya male and hermaphrodite flowers at early stages, suggesting that this gene might participate in male flower organ development processes, nevertheless, this gene cannot be considered a sex-determination gene. Due to its homology with other plant MS1 proteins and its expression pattern, we hypothesize that this gene participates in anther development processes, like tapetum and pollen development, downstream gender specification. Further gene functional characterization studies in papaya are required to confirm this hypothesis. The role of ABA and ROS signaling pathways in papaya flower development needs to be further explored as well.
Highlights
Carica papaya is a trioecious plant species with a genetic sex-determination system defined by sex chromosomes
The aim of this study is to identify genes that are differentially expressed among male, female and hermaphrodite flower buds in papaya during early and later stages of flower development using Ribonucleic Acid (RNA)-seq, and to evaluate the expression of highly differentially expressed genes by RT-qPCR, as well as to identify the putative functions for these genes based on their homology with other plant species and their expression patterns
Differential expression in the anther development pathway The major finding of this study was a Male Sterility 1 gene (CpMS1) highly up-regulated in male and hermaphrodite flower buds compared to female flower buds, with tissue and developmental specific expression
Summary
Carica papaya is a trioecious plant species with a genetic sex-determination system defined by sex chromosomes. Unisexual flowers in angiosperm plant species are classified as monoecious or dioecious. Dioecious plant species could evolve from hermaphroditic or monoecious populations in three major steps [1, 2]. A recessive male-sterile mutation occurred originating female plants. The occurrence of this mutation would be advantageous for the population, because female plants could be pollinated by individuals from different populations, reducing the inbreeding and increasing the genetic variability within the population. A second dominant female-sterile mutation appeared in the monoecious population generating male plants. The pair of chromosomes in which these mutations occurred stopped recombining and started accumulating mutations and repetitive elements. Recombination stopped because an individual with both mutations would become completely sterile, representing no advantage for the population. The chromosomes carrying these mutations became a pair of different sex chromosomes [1, 2]
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