Retroelement expansions underlie genome evolution in stingless bees

Abstract

Stingless bees are essential pollinators and emerging models for studying behavioral and genomic evolution. In the genus Melipona, a major difference in heterochromatin organization defines two groups: Group I species (e.g., M. quadrifasciata) with 50% heterochromatin across their chromosomes. These differences are thought to correlate with genome size and transposable element (TE) content, offering a unique opportunity to explore how heterochromatin variation, TE dynamics, and chromosomal evolution interact in a phylogenetic context. We present pseudo-chromosome-level genome assemblies for M. quadrifasciata and M. scutellaris obtained by long-read sequencing and Hi-C scaffolding. Comparative analyses reveal conserved synteny but marked divergence in structural variants and TEs. M. scutellaris shows an expansion of retrotransposons, particularly Gypsy/DIRS1 elements, concentrated in TE hotspots linked to chromosomal rearrangements and structural variants. This coincides with distinct methylation entropy and an expansion of histone deacetylase orthologs, potentially affecting heterochromatin organization. The increased ratio of retrotransposons in M. scutellaris is counterbalanced by more DNA transposons in M. quadrifasciata, resulting in genomes of similar overall sizes but of distinct heterochromatin distribution. Advancing our understanding of genome evolution in eusocial insects, we provide high-resolution genomic resources for two Melipona species that differ in heterochromatin content. Our results highlight the complex role of TEs in shaping genomes and underscore their influence on chromosomal and epigenetic innovation, providing strong evidence that TE dynamics underly the striking heterochromatic differences observed in Melipona.