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Chromosome Evolution Model Reveals Hidden Variation in Fern Diversification Rates
Thomas D Buchloh1,2, Carrie M Tribble3, Michael R May4
1Department of Biological Sciences, Clemson University, Clemson, SC 29634, United States.
Systematic Biology
|July 22, 2026
Summary
Changes in chromosome number (karyotype) can drive speciation. This study reveals that highly variable karyotype evolution in ferns may decrease long-term lineage persistence, challenging previous assumptions about diversification rates.
Area of Science:
- Evolutionary Biology
- Genetics
- Botany
Background:
- Changes in chromosome number (karyotype) are a known driver of reproductive isolation and speciation.
- The relationship between karyotype evolution rates and diversification rates is well-established in some plant groups, but less understood in ferns.
- Previous methods for analyzing chromosome number evolution are computationally intensive, limiting their application to smaller clades.
Purpose of the Study:
- To investigate the mode and tempo of karyotype evolution in the diverse fern order Polypodiales.
- To extend the Chromosome Number and Hidden State-dependent Speciation and Extinction (ChromoHiSSE) model to incorporate whole genome duplication.
- To estimate rates of karyotype evolution and identify hidden modes of chromosome number change in ferns.
Main Methods:
- Utilized an extended ChromoHiSSE model incorporating whole genome duplication.
- Analyzed karyotype evolution across 962 species of leptosporangiate ferns within the Polypodiales.
- Employed stochastic mapping to trace the historical occurrence of different karyotype evolution modes.
Main Results:
- Identified two distinct hidden modes of chromosome number evolution with rates differing by over an order of magnitude.
- Found that lineages exhibiting high karyotype lability are less likely to persist over long evolutionary timescales.
- Polyploid speciation significantly shapes modern fern diversity.
Conclusions:
- While polyploidy is a key factor in fern diversification, rapid karyotype evolution does not necessarily enhance long-term lineage survival.
- High karyotype lability may be associated with decreased long-term persistence, contrary to expectations that chromosome number changes boost diversification.
- The study provides new insights into the complex interplay between karyotype evolution, speciation, and diversification in ferns.
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