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In vascular plants, heterosporous lineages typically have fewer chromosomes than homosporous lineages. The underlying mechanism causing this disparity has been debated for over half a century. Although reproductive mode has been identified as critical to these patterns, the symmetry of meiosis during sporogenesis has been overlooked as a potential cause of the difference in chromosome numbers. In most heterosporous plants, meiosis during megasporogenesis is asymmetric, meaning one of the four meiotic products survives to become the egg. Comparatively, meiosis is symmetric in homosporous megasporogenesis and all meiotic products survive. The symmetry of meiosis is important because asymmetric meiosis enables meiotic drive and associated genomic changes, while symmetric meiosis cannot lead to meiotic drive. Meiotic drive is a deviation from Mendelian inheritance where genetic elements are preferentially inherited by the surviving egg cell, and can profoundly impact chromosome (and genome) size, structure, and number. Here we review how meiotic drive impacts chromosome number evolution in heterosporous plants, how the lack of meiotic drive in homosporous plants impacts their genomes, and explore future approaches to understand the role of meiotic drive on chromosome number across land plants. 

A longstanding question in plant evolution is why ferns have many more chromosomes than angiosperms. The leading hypothesis proposes that ferns have ancient polyploidy without chromosome loss or gene deletion to explain the high chromosome numbers of ferns. Here, we test this hypothesis by estimating ancient polyploidy frequency, chromosome evolution, protein evolution in meiosis genes, and patterns of gene retention in ferns. We found similar rates of paleopolyploidy in ferns and angiosperms from independent phylogenomic and chromosome number evolution analyses, but lower rates of chromosome loss in ferns. We found elevated evolutionary rates in meiosis genes in angiosperms, but not in ferns. Finally, we found some evidence of parallel and biased gene retention in ferns, but this was comparatively weak to patterns in angiosperms. This work provides genomic evidence supporting a decades-old hypothesis on fern genome evolution and provides a foundation for future work on plant genome structure.