Sex Determination: Sterility Genes out of Sequence

Why should we study plant sex chromosomes?

Understanding plant sex chromosomes involves studying interactions between developmental and physiological genetics, genome evolution, and evolutionary ecology. We focus on areas of overlap between these. Ideas about how species with separate sexes (dioecious species, in plant terminology) can evolve are even more relevant to plants than to most animal taxa because dioecy has evolved many times from ancestral functionally hermaphroditic populations, often recently. One aim of studying plant sex chromosomes is to discover how separate males and females evolved from ancestors with no such genetic sex-determining polymorphism, and the diversity in the genetic control of maleness vs femaleness. Different systems share some interesting features, and their differences help to understand why completely sex-linked regions may evolve. In some dioecious plants, the sex-determining genome regions are physically small. In others, regions without crossing over have evolved sometimes extensive regions with properties very similar to those of the familiar animal sex chromosomes. The differences also affect the evolutionary changes possible when the environment (or pollination environment, for angiosperms) changes, as dioecy is an ecologically risky strategy for sessile organisms. Dioecious plants have repeatedly reverted to cosexuality, and hermaphroditic strains of fruit crops such as papaya and grapes are desired by plant breeders. Sex-linked regions are predicted to become enriched in genes with sex differences in expression, especially when higher expression benefits one sex function but harms the other. Such trade-offs may be important for understanding other plant developmental and physiological processes and have direct applications in plant breeding.

Remarkable Divergence of the Sex-Linked Region between Two Wild Spinach Progenitors, Spinacia turkestanica and Spinacia tetrandra

The sex-linked region (SLR) plays an important role in determining the sex of a plant. The SLR of the Y chromosome, composed of a 14.1-Mb inversion and a 10-Mb Y-duplication region (YDR), was deciphered in Spinacia oleracea previously. However, our understanding of the SLR in its wild relatives, S. turkestanica and S. tetrandra, remains limited. In this study, we used 63 resequencing data from the three Spinacia species to infer the evolution of the SLR among the Spinacia species. In the SLR, all the cultivated spinach and S. turkestanica accessions were clustered into two distinct categories with both sexes, while the S. tetrandra accessions of both sexes were grouped. This suggests that S. oleracea shared a similar SLR with S. turkestanica, but not with S. tetrandra, which was further confirmed based on the population structure and principal component analysis. Furthermore, we identified 3910 fully sex-linked SNPs in S. oleracea and 92.82% of them were available in S. turkestanica, while none of the SNPs were adopted in S. tetrandra. Genome coverage in males and females supported the hypothesis that the YDR increasingly expanded during its evolution. Otherwise, we identified 13 sex-linked transposable element insertion polymorphisms within the inversion in both S. oleracea and S. turkestanica, demonstrating that the transposable element insertions might have occurred before the recombination suppression event of the inversion. The SLR was conserved compared with the pseudoautosomal region given that the genetic hitchhiking process occurred in the SLR during its evolution. Our findings will significantly advance our understanding of the characteristics and evolution of the SLR in Spinacia species.

Love is in the air: ethylene and sex determination in Cucurbita pepo

This article comments on:

García A, Aguado E, Martínez C, Loska D, Beltrán S, Valenzuela JL, Garrido D, Jamilena M. 2019. The ethylene receptors CpETR1A and CpETR2B cooperate in the control of sex determination in Cucurbita pepo. Journal of Experimental Botany 70, 154–167.

Anatomy and RNA-Seq reveal important gene pathways regulating sex differentiation in a functionally Androdioecious tree, Tapiscia sinensis

BACKGROUND

Gametogenesis is a key step in the production of ovules or pollen in higher plants. The sex-determination aspects of gametogenesis have been well characterized in the model plant Arabidopsis. However, little is known about this process in androdioecious plants. Tapiscia sinensis Oliv. is a functionally androdioecious tree, with both male and hermaphroditic individuals. Hermaphroditic flowers (HFs) are female-fertile flowers that can produce functional pollen and set fruits. However, compared with male flowers (MFs), the pollen viability and number of pollen grains per flower are markedly reduced in HFs. MFs are female-sterile flowers that fail to set fruit and that eventually drop.

RESULTS

Compared with HF, a notable cause of MF female sterility in T. sinensis is when the early gynoecium meristem is disrupted. During the early stage of HF development (stage 6), the ring meristem begins to form as a ridge around the center of the flower. At this stage, the internal fourth-whorl organ is stem-like rather than carpelloid in MF. A total of 52,945 unigenes were identified as transcribed in MF and HF. A number of differentially expressed genes (DEGs) and metabolic pathways were detected as involved in the development of the gynoecium, especially the ovule, carpel and style. At the early gynoecium development stage, DEGs were shown to function in the metabolic pathways regulating ethylene biosynthesis and signal transduction (upstream regulator), auxin, cytokinin transport and signalling, and sex determination (or flower meristem identity).

CONCLUSIONS

Pathways for the female sterility model were initially proposed to shed light on the molecular mechanisms of gynoecium development at early stages in T. sinensis.