The authors performed a comparative genomic analysis spanning 51 spider species to investigate genetic correlates of extreme female-biased sexual size dimorphism (SSD). This taxon offers an exceptional natural experiment because orb-weaving spiders include some of the most pronounced cases of female-biased SSD among terrestrial arthropods. Across these species, the study identified evolutionary-rate shifts in sets of genes associated with chromatin regulation, growth-related pathways, and structural components of the cuticle.
These genome-scale comparisons highlighted categories of genes whose evolutionary dynamics covary with the degree of SSD, suggesting that divergence in gene sequence evolution may contribute to the phenotypic differences between sexes seen in strongly dimorphic lineages.
To link interspecific genomic patterns with within-species regulatory mechanisms, the researchers carried out sex-resolved transcriptomic and chromatin profiling in the highly dimorphic Joro spider, Trichonephila clavata. The approach combined expression data with assays of chromatin accessibility and local chromatin interactions to capture regulatory differences between males and females at multiple levels.
This species-level multi-omic profiling was used to test whether genes and pathways implicated by the comparative genomics showed corresponding sex-specific regulatory patterns in a species that exhibits extreme female-biased body-size dimorphism.
The cross-species analysis detected SSD-associated shifts in evolutionary rates for genes functioning in chromatin regulation, growth control, and cuticle structural organization. Such shifts suggest that both the regulation of gene expression (via chromatin) and the molecular machinery governing growth and external morphology (cuticle) have experienced altered selective pressures in lineages with pronounced female-biased SSD.
The identification of chromatin-related genes among those with altered evolutionary dynamics supports the idea that changes in gene regulatory architecture accompany or facilitate extreme sexual dimorphism.
A prominent genomic correlate reported was a significant positive association between copy number of the juvenile hormone acid O-methyltransferase gene (JHAMT) and the magnitude of SSD across the sampled spider species. Lineages with more extreme female-biased SSD tended to have larger repertoires of JHAMT paralogs.
This copy-number relationship implicates expansion of a hormone-related enzyme gene family as a candidate genomic change linked to the evolution of body-size differences between the sexes.
Within Trichonephila clavata, the expanded JHAMT paralogs showed sex- and tissue-biased expression. These expression patterns were accompanied by sex-specific differences in chromatin accessibility and in local chromatin interactions at those loci.
Together, these observations indicate that both gene-family expansion and sex-specific regulatory states (transcriptional and chromatin-level) are associated with the JHAMT paralogs in a species that exhibits extreme SSD.
An integrated analysis combining transcriptional data, chromatin accessibility profiles, and chromatin interaction maps revealed coordinated sex-specific differences across multiple regulatory layers. The affected gene sets include components of ecdysteroid signaling, insulin and growth regulation pathways, and genes involved in epidermal and cuticular processes.
These coordinated, multi-omic divergences imply that transcriptional programs, chromatin state, and three-dimensional chromatin interactions act in concert to produce sex-specific developmental and physiological outcomes tied to body size and cuticular morphology.
By merging comparative genomics with sex-resolved multi-omic profiling in a highly dimorphic species, the study links evolutionary-rate shifts and gene-family expansion to sex-specific regulatory divergence. Key findings include accelerated or shifted evolution in chromatin, growth, and cuticle genes across spiders; a positive correlation between JHAMT copy number and SSD; and sex-specific expression and chromatin features of expanded JHAMT paralogs in Trichonephila clavata. Integrated multi-omic signatures further implicate ecdysteroid, insulin/growth, and epidermal/cuticular pathways in female-biased SSD.
Collectively, these results provide a multi-level genomic and regulatory framework connecting evolutionary change to the extreme female-biased body-size dimorphism observed in orb-weaving spiders. The authors report no competing interests. Specific sample sizes, statistical values, and detailed gene lists are presented in the source article's full text and supplementary material but are not reproduced here beyond the summary findings.