Microchromosomes

Microchromosomes are extremely small. Often they cannot be determined in the karyotype by cytogenetic methods, and their presence significantly complicates the whole process of studying the karyotype. Initially, they were considered as insignificant fragments of chromosomes, however, in the studied species with microchromosomes, it was found that they carry many genes . So, in chicken ( Gallus gallus ), microchromosomes account for 50% to 75% of all genes ^{[4] [5]} . In the metaphase, microchromosomes are visible as tiny specks 0.5-1.5 microns long. Their small size, as well as the small degree of condensation into heterochromatin, determine the inapplicability of standard cytogenetic methods for identifying chromosomes ^[1] .

In most birds (with the exception of representatives of falconiform and some other species), the karyotype contains about 80 chromosomes ( 2 n ≈ 80), of which well distinguishable macrochromosomes make up only a small part, and more than 60 chromosomes are microchromosomes ^[1] . Birds have a greater number of microchromosomes than any other group of animals . Chicken is the most convenient object for the analysis of microchromosomes ^[1] . The study of bird microchromosomes showed that they could arise as conservative fragments of ancestral macrochromosomes, or, on the contrary, macrochromosomes arose as aggregates of microchromosomes ^[1] . Comparative genomic analysis showed that microchromosomes contain genetic information that is conservative for many classes of chromosomes. It was shown that at least 10 chicken microchromosomes arose as a result of cleavage of macrochromosomes and that a typical bird karyotype formed 100-250 million years ago ^[5] .

The refinement of physical maps of avian microchromosomes is carried out using the in situ fluorescence hybridization method; in this case, cloned DNA sequences are used as DNA probes (for example, Not I-binding clones of the 3rd chromosome of the human , -clones) ^{[6] [7]} . Further progress in the identification and investigation of microchromosomes (including those involved in their rearrangements ) is associated with the use of chicken microchromosome probes for the so-called “ chromosome painting ” ^[8] , pairs of specific LHC chicken clones for each microchromosome and bioinformatics methods for genomic comparisons of in silico microchromosomes ^[9] .

Chicken

In chicken, the chromosome set is 78 chromosomes (2 n = 78), and, as in the case of all birds, most chromosomes are microchromosomes. According to earlier views, among chicken chromosomes there are 6 pairs of macrochromosomes, one pair is sex chromosomes and 32 pairs are intermediate chromosomes or microchromosomes ^[4] . According to a later description of the chicken karyotype, it is formed of 5 pairs of macrochromosomes, one pair of sex chromosomes, 5 pairs of intermediate chromosomes and 28 pairs of microchromosomes ^{[3] [10]} . Microchromosomes make up about a third of the entire genome and exhibit a higher gene density than macrochromosomes. In this regard, it is assumed that most of the genes are localized on microchromosomes ^[5] , although due to the complex physical identification of microchromosomes and the inability to use microsatellite markers, it is difficult to establish the exact position of the gene on a particular microchromosome ^[10] .

Between macrochromosomes and chicken microchromosomes, differences in replication synchronization and recombination rates were detected. In the S phase of interphase, microchromosomes are replicated before macrochromosomes ^[4] . Recombination rates are also higher for microchromosomes ^[11] . Possibly, due to the high recombination rate, the chicken chromosome 16 (microchromosome), as it was found, has the greatest variety of genes than any other chromosome in a number of chicken breeds ^[11] . This is presumably related to the localization of the genes of the main histocompatibility complex on this chromosome.

For many cohesion groups whose position on specific chromosomes has not been established, it is assumed that they are localized on microchromosomes. Interestingly, these linkage groups correspond to large sections of human chromosomes. Thus, the linkage groups E29C09W09, E21E31C25W12, E48C28W13W27, E41W17, E54 and E49C20W21 correspond to the 7th human chromosome ^[10] .

Turkey

The turkey chromosome set includes 80 chromosomes (2 n = 80). The presence of an additional, compared to chickens, chromosome pair is due to at least two chromosomal rearrangements , which are splitting / fusion of individual sections (GGA2 = MGA3 + MGA6 and GGA4 = MGA4 + MGA9). Other rearrangements were revealed in a comparative analysis of genetic ^[12] and physical maps, as well as sequencing of the complete genome ^[13] .

Microchromosomes are absent in the karyotypes of humans , crocodiles and frogs ^[1] .

In rare cases, microchromosomes are detected in karyotypes of individuals. A relationship was established between the presence of microchromosomes and genetic abnormalities such as Down Syndrome ^[14] and Martin – Bell Syndrome ^[15] . Normally, the smallest human autosome — the 21st — is 47 megabytes.