Highly Conserved Microchromosomal Organization in Passeriformes Birds Revealed via BAC-FISH Analysis

Simple Summary The Passeriformes order (songbirds) is incredibly diverse in terms of number of species and morphological and ecological diversification, comprising around 60% of all bird species. Despite considerable diversity, the genome organizational structure (i.e., the number and pattern of chromosomes) within Passeriformes is highly conserved, with a chromosome number that remains close to 80 in nearly all species studied. These characteristics raise interesting questions and stimulate curiosity about the genome evolution of this group. Therefore, this study aimed to analyze the organization of the smallest chromosomes (microchromosomes) in four Passeriformes species to understand whether they were rearranged during evolution. This has only recently become possible using fluorescent probes called bacterial artificial chromosomes (BACs) and a technique called fluorescence in situ hybridization (FISH). Our results confirm that the songbirds studied did not rearrange their microchromosomes to any great extent, and this may have contributed to their overall evolutionary success.Abstract Passeriformes birds are widely recognized for their remarkable diversity, with over 5700 species described so far. Like most bird species, they possess a karyotype characteristic of modern birds, which includes a bimodal karyotype consisting of a few pairs of macrochromosomes and many pairs of microchromosomes. Although the karyotype is typically 2n = 80, the diploid number can atypically vary greatly, ranging from 56 to approximately 100 chromosomes. In this study, we aimed to understand the extent of conservation of the karyotype's organizational structure within four species of this group using Bacterial Artificial Chromosomes via Fluorescence In Situ Hybridization (BAC-FISH) with microchromosome probes from Chicken (Gallus gallus) or Zebra Finch (Taeniopygia guttata) per microchromosomes (GGA10-28, except GGA16). By examining the chromosome complement of four passerine species-the Streaked Flycatcher (Myiodynastes maculatus), Shiny Cowbird (Molothrus bonariensis), Southern House Wren (Troglodytes aedon), and Double-collared Seedeater (Sporophila caerulescens)-we discovered a new chromosome number for Southern House Wren. Through FISH experiments, we were able to observe the same pattern of microchromosome organization as in the common ancestor of birds. As a result, we propose a new diploid number for Southern House Wren and confirm the conservation status of microchromosome organization, which may confer evolutionary advantages to this group.