# Transcriptomes of plant gametophytes have a higher proportion of rapidly evolving and young genes than sporophytes

## TL;DR

This paper shows that plant reproduction involves a surge in young, diverse genes, differing from other developmental stages.

## Contribution

The study introduces the concept of an 'evolutionary bulge' in plant reproductive development, contrasting it with the transcriptomic hourglass model.

## Key findings

- A significant shift in gene expression occurs during gametogenesis with younger genes contributing more.
- The evolutionary bulge pattern is stronger than the transcriptomic hourglass in plant reproduction.
- The hourglass pattern was observed only in Arabidopsis thaliana, not in rice or soybean.

## Abstract

Reproductive traits in plants tend to evolve rapidly due to various causes that include plant-pollinator coevolution and pollen competition, but the genomic basis of reproductive trait evolution is still largely unknown. To characterise evolutionary patterns of genome wide gene expression in reproductive tissues in the gametophyte and to compare them to developmental stages of the sporophyte, we analysed evolutionary conservation and genetic diversity of protein-coding genes using microarray-based transcriptome data from three plant species, Arabidopsis thaliana, rice (Oryza sativa) and soybean (Glycine max). In all three species a significant shift in gene expression occurs during gametogenesis in which genes of younger evolutionary age and higher genetic diversity contribute significantly more to the transcriptome than in other stages. We refer to this phenomenon as “evolutionary bulge” during plant reproductive development because it differentiates the gametophyte from the sporophyte. We show that multiple, not mutually exclusive, causes may explain the bulge pattern, most prominently reduced tissue complexity of the gametophyte, a varying extent of selection on reproductive traits during gametogenesis as well as differences between male and female tissues. This highlights the importance of plant reproduction for understanding evolutionary forces determining the relationship of genomic and phenotypic variation in plants.