Why Do Heterosporous Plants Have So Few Chromosomes?

Sylvia P Kinosian,Paul G Wolf,Carol A Rowe

doi:10.3389/fpls.2022.807302

Sylvia P Kinosian, Paul G Wolf + Show 1 more

Open Access

https://doi.org/10.3389/fpls.2022.807302

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Abstract

The mechanisms controlling chromosome number, size, and shape, and the relationship of these traits to genome size, remain some of the least understood aspects of genome evolution. Across vascular plants, there is a striking disparity in chromosome number between homosporous and heterosporous lineages. Homosporous plants (comprising most ferns and some lycophytes) have high chromosome numbers compared to heterosporous lineages (some ferns and lycophytes and all seed plants). Many studies have investigated why homosporous plants have so many chromosomes. However, homospory is the ancestral condition from which heterospory has been derived several times. Following this phylogenetic perspective, a more appropriate question to ask is why heterosporous plants have so few chromosomes. Here, we review life history differences between heterosporous and homosporous plants, previous work on chromosome number and genome size in each lineage, known mechanisms of genome downsizing and chromosomal rearrangements, and conclude with future prospects for comparative research.

Highlights

The nuclear genetic material of eukaryotes is contained within chromosomes
We address the current evidence for the processes controlling genome evolution and the variation in genome downsizing rates between homosporous and heterosporous plants, as well as how the pattern of spore production might be related to these mechanisms
If we are to understand the relationship between genome architecture and spore type, the necessary studies must begin with comparative analyses from a phylogenetic perspective

Summary

INTRODUCTION

The nuclear genetic material of eukaryotes is contained within chromosomes. The number, length, and centromere location of chromosomes in an organism (the karyotype) varies considerably among lineages (Ohno, 1984; Schubert and Lysak, 2011). Some studies suggest that genome size is positively correlated with chromosome number in homosporous ferns (Nakazato et al, 2008; Bainard et al, 2011; Clark et al, 2016; Fujiwara et al, 2021), indicating that ferns have fundamentally different mechanisms of genome downsizing compared to other organisms (Barker and Wolf, 2010; Leitch and Leitch, 2012). Work in angiosperms has found that mitotic divisions may fail when the chromosome arm: spindle length ratio is above a certain point (Schubert and Oud, 1997) Investigating this ratio in homosporous and heterosporous ferns and lycophytes could be a fruitful avenue of study, as there may be fundamental differences in cell division between these lineages and seed plants. Overall, investigating patterns of both chromosome- and genome-scale structure will greatly benefit our understanding of both heterosporous and homosporous genomes

DISCUSSION

CONCLUSION