microchromosome

Microchromosome is a term used in genetics to describe very small chromosomes that are distinct from the larger macrochromosomes in a genome. They are most commonly discussed in birds and many reptiles, where genomes contain a large number of these tiny chromosomes. Microchromosomes are generally much shorter than macrochromosomes and can carry a substantial portion of the organism’s genes.

Several characteristic features distinguish microchromosomes from macrochromosomes. They tend to be gene-dense, with a high concentration of coding sequences and CpG-rich regions. They also exhibit high GC content and often show elevated recombination rates compared with larger chromosomes. This combination of gene density and high recombination has been linked to particular patterns of genome evolution, including GC-biased gene conversion. In many species, microchromosomes appear to form a distinct nuclear domain and may replicate early in S phase, though replication timing can vary across taxa.

The distribution and evolution of microchromosomes vary among lineages. In birds, microchromosomes are a prominent and relatively conserved feature of the genome, while some reptile lineages show variation in the number and size of microchromosomes due to chromosomal fusions and rearrangements. Comparative genomics indicates that macrochromosomes in at least some groups may originate in part from the fusion of ancestral microchromosomes.

Research on microchromosomes relies on cytogenetic techniques such as karyotyping and fluorescence in situ hybridization, as well as modern genome sequencing and assembly methods. Understanding their structure and function contributes to broader insights into genome organization, evolution, and gene regulation.