Genome size evolution in Helianthemum (Cistaceae): Dynamic genomes within a conserved chromosomal framework.

Area of Science:

• Plant biology
• Evolutionary biology
• Genomics

Background:

• Genome size is a key biological trait, but its evolution is poorly understood in plants due to limited cytogenetic and phylogenomic data.
• Few plant genera offer sufficient data for genus-level genome size evolution studies.

Purpose of the Study:

• To investigate genome size (2C values) evolution across 80 Helianthemum species, representing 75% of the genus.
• To analyze genome size evolution within a new phylogenetic framework and integrate karyotype descriptors.
• To understand the factors driving genome size diversification in Helianthemum.

Main Methods:

• Estimated genome size (2C values) for 80 Helianthemum species.
• Generated a time-calibrated phylogenetic framework using the Angiosperms353 probe set.
• Integrated karyotype data: chromosome numbers, total haploid karyotype length (THL), interchromosomal coefficient of variation in chromosome length (CVCL), and intrachromosomal mean centromeric asymmetry (MCA).
• Performed phylogenetically informed regressions and ancestral-state reconstructions.

Main Results:

• Genome size varied 6.5-fold (1.65 to 10.60 pg) across the genus.
• A strong positive correlation was found between genome size (2C values) and total haploid karyotype length (THL).
• Genome size was not significantly associated with chromosome number or karyotype asymmetry.
• Karyotype was highly conserved, with minimal chromosomal rearrangements.
• Repeated genome size reductions from an intermediate ancestral genome were observed, shaped by phylogenetic structure.

Conclusions:

• Helianthemum exhibits significant genome size variation primarily driven by changes in chromosome size, not number.
• Genome size evolution in Helianthemum occurs largely independently of major chromosomal rearrangements.
• The genus provides a valuable model for studying genome size evolution within a conserved chromosomal context.