Abstract
It is old-established that the chromosome number in the chimpanzee, gorilla and orangutan was 48 and not 46 as in man. However, cause and effect of such chromosome rearrangement is unknown. A hypothesis has been proposed that natural selection caused merger of two pairs of autosomes into one chromosome. In the changed climate of the East Africa individuals with less amount of chromosomal Q-heterochromatin regions (Q-HRs) in genome were the most adapted. Two pairs of acrocentrics in the genome the common ancestor, which merged into a single chromosome, apparently, carried on their short arms of Q-HRs with a very high frequency, preventing the birth of individuals with a low number Q-heterochromatin. With the merger of these two pairs of acrocentrics into one, the number of autosomes bearing the Q-HRs reduced from nine to seven pairs, as in the modern human. Such chromosome rearrangement resulted in two important consequences: а) chromosomal Q-HRs distributed into seven Q-polymorphic autosomes, so that it was possible to give birth to the individuals with different, including the low, number of Q-heterochromatin; b) in the population individuals with low number of Q-HRs appeared, able to adapt to new, harsher climatic conditions. With the lapse of time, these individuals formed a new population in the new territory, where individuals with a number of chromosomal Q-HRs like the modern natives of Africa, and with the number of 46 chromosomes in the genome began to dominate. Thus, the cause of the origin of the 46 chromosome karyotype from an ancestral 48 chromosome line was natural selection, and an effect was adaptation, i.e. individuals with different, including the low, number of Q-HRs, got the advantage to open up and to colonize new ecological zones of the East Africa.
Highlights
The modern techniques of differential chromosomal staining (G, C, Q- and R-staining) established that the chromosome number in the chimpanzee, gorilla and orangutan was 48 and not 46 as in man
The range of opinions is quite wide. They can be summarized in two extreme opinions: humans appeared after the diploid number of chromosomes in its karyotype became equal to 46, instead of 48, or became 46, after the modern human had formed
In our opinion the most important, in aspect we are interested in, the techniques of chromosomal C- and Q-staining, used for study of heterochromatin areas (HRs) of chromosomes, can give the information. By means of these methods the facts are collected, which, as we hope, can help to construct a hypothesis about an origin of the modern human with the 46 chromosomes. These facts are the follows: 1) Heterochromatin is universally distributed in the chromosomes of all the eukaryotes - plants, animals and man, accounting for 10% to 60% of their genome
Summary
The modern techniques of differential chromosomal staining (G-, C-, Q- and R-staining) established that the chromosome number in the chimpanzee, gorilla and orangutan was 48 and not 46 as in man. In our opinion the most important, in aspect we are interested in, the techniques of chromosomal C- and Q-staining, used for study of heterochromatin areas (HRs) of chromosomes, can give the information By means of these methods the facts are collected, which, as we hope, can help to construct a hypothesis about an origin of the modern human with the 46 chromosomes. We display here some the additional facts, concerning peculiarities of distribution of chromosomal Q-HRs in the genome of populations of the modern human It is established: 1) Q-HRs is detected on certain loci of only seven autosomes (3, 4, 13, 14, 15, 21 and 22) in both sexes, as well as on the Y chromosome of males. Despite the presence of the large Y chromosome in the orangutan intensely fluorescent chromatin is not found in the species or elsewhere in the primate and mammalian kingdom indicating that it must have had a relatively recent origin, that man, chimpanzee, and gorilla form a natural group and that they have had a recent common ancestor (Pearson et al, 1971)
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