Microchromosome

A microchromosome is a chromosome defined for its relatively small size. They are typical components of the karyotype of birds, some reptiles, fish, amphibians, and monotremes.^[1] As many bird genomes have chromosomes of widely different lengths, the name was meant to distinguish them from the comparatively large macrochromosomes.^[2] The distinction referred to the measured size of the chromosome while staining for karyotype, and while there is not a strict definition, chromosomes resembling the large chromosomes of mammals were called macrochromosomes (roughly 3 to 6 µm), while the much smaller ones of less than around 0.5 µm were called microchromosomes.^[3] In terms of base pairs, by convention, those of less than 20Mb were called microchromosomes, those between 20 and 40 Mb are classified as intermediate chromosomes, and those larger than 40Mb are macrochromosomes.^[4] By this definition, all normal chromosomes in organisms with relatively small genomes (less than 100-200Mb) would be considered microchromosomes.

Function

Microchromosomes are characteristically very small and often

Replication timing and recombination rates have been found to differ between micro- and macrochromosomes in chickens. Microchromosomes replicate earlier in the S phase of interphase than macrochromosomes.^[5] Recombination rates have also been found to be higher on microchromosomes.^[9] Possibly due to the high recombination rates, chicken chromosome 16 (a microchromosome) has been found to contain the most genetic diversity of any chromosome in certain chicken breeds.^[9] This is likely due to the presence on this chromosome of the major histocompatibility complex (MHC).

For the many small linkage groups in the chicken genome which have not been placed on chromosomes, the assumption has been made that they are located on the microchromosomes. Groups of these correspond almost exactly with large sections of certain human chromosomes. For example, linkage groups E29C09W09, E21E31C25W12, E48C28W13W27, E41W17, E54 and E49C20W21 correspond with

The turkey has a diploid number of 80 (2n = 80) chromosomes. The karyotype contains an additional chromosomal pair relative to the chicken due to the presence of at least two fission/fusion differences (GGA2 = MGA3 and MGA6 and GGA4 = MGA4 and MGA9). Given these differences involving the macrochromosomes, an additional fission/fusion must also exist between the species involving the microchromosomes if the diploid numbers are valid. Other rearrangements have been identified through comparative genetic maps,^[10] physical maps and whole genome sequencing.^[11]

In turtles

Microchromosomes play a key role in sex determination in soft-shelled turtles.^[12]

In humans and other animals

Microchromosomes are absent from the karyotypes of

• Minichromosome

References