Microchromosomes

Microchromosomes are a class of very small chromosomes found in the karyotypes of several vertebrate groups, most prominently birds and many reptiles. Unlike macrochromosomes, microchromosomes are relatively tiny in size and have distinct sequence and chromatin features.

Size and content: They typically range from a few hundred kilobases up to several tens of megabases, depending on the species. They are often gene-rich for their size, with high GC content and short intergenic regions, leading to high gene density.

Genomic features: Higher recombination rates per base pair, open chromatin, and early replication timing are commonly observed in microchromosomes. They are enriched for housekeeping and other essential genes, contributing to robust gene expression and genetic diversity in the regions they encode.

Nuclear organization and evolution: In birds, microchromosomes often occupy a distinct central nuclear compartment and can cluster together. Evolutionarily, they are thought to reflect ancient chromosomal fragments retained as small units; many vertebrate lineages retain such sets, while others, including most mammals, lack a prominent microchromosome component.

Taxonomic distribution and significance: Microchromosomes are most characteristic of birds and many reptiles, with some amphibians and fish also retaining microchromosome sets. Their study informs understanding of genome architecture, evolution, and gene regulation, and can influence approaches to genome sequencing and assembly in species where they are prominent.