A phylogenetic perspective of chromosome evolution in Formicidae

Chromosomes, as carriers of genes, are the fundamental units of heredity, with the eukaryotic genome divided into multiple chromosomes. Each species typically has a consistent number of chromosomes within its lineage. Ants, however, display remarkable diversity in chromosome numbers, and previous studies have shown that this variation may correlate with ant diversity. As ants evolved, various karyotypes emerged, primarily through chromosomal fissions, leading to an increase in chromosome number and a decrease in chromosome size. In this study, we investigate chromosome evolution in ants from a phylogenetic perspective using ancestral reconstruction. Our analysis indicates that the most recent common ancestor of ants had an ancestral haploid chromosome number of 11, likely composed of biarmed chromosomes. The bimodal distribution of karyotypes and the trend toward increased chromosome numbers align with previous assumptions. However, both dysploidy and ploidy changes have been indicated as likely mechanisms of chromosome number evolution. Descending dysploidy occurs consistently throughout the phylogeny, while changes in ploidy are believed to occur occasionally within the subfamilies during genus diversification. We propose, based on our results and previous evidence (e.g., genome size in ants), that both fusions and fissions contribute equally to karyotype changes in Formicidae. Additionally, changes in ploidy should not be fully ignored, as they can occur across specific lineages.