Survival of Chromosomal Changes

Survival of Chromosomal Changes

SURVIVAL of CHROMOSOMAL CHANGES The following are excerpts from the informative blog by Robert Williams, specifically from his... Comparison of the Human and Great Ape Chromosomes as Evidence for Common Ancestry ... which is part of his larger collection as...Evidence for Human and Ape Common Ancestry.

HOW CHROMOSOMAL CHANGES CAN SURVIVE and LEAD TO NEW SPECIES Some may raise the objection that if the fusion was a naturalistic event, how could the first human ancestor with the fusion have successfully reproduced? We have all heard that the horse and the donkey produce an infertile mule in crossing because of a different number of chromosomes in the two species. Well, apparently there is more to the story than we are usually told, because variations in chromosome number are known to occur in many different animal species, and although they sometimes seem to lead to reduced fertility, this is often not the case. Refs 5, 6, and 7 document both the existence of such chromosomal number differences and the fact that differences do not always result in reduced fertility. I can provide many more similar references if required. The last remaining species of wild horse, Przewalski's (sha-val-skis) Wild Horse has 66 chromosomes while the domesticated horse has 64 chromosomes. Despite this difference in chromosome number, Przewalski's Wild Horse and the domesticated horse can be crossed and do produce fertile offspring (see reference 9 below).

5. Chromosomal heterozygosity and fertility in house mice (Mus musculus domesticus) from Northern Italy. Hauffe HC, Searle JB. Department of Zoology, University of Oxford, Oxford OX1 3PS, United Kingdom.

Following the discovery of over 40 Robertsonian (Rb) races of Mus musculus domesticus in Europe and North Africa, the house mouse has been studied extensively as an ideal model to determine the chromosomal changes that may cause or accompany speciation. Current models of chromosomal speciation are based on the assumption that heterozygous individuals have a particularly low fertility, although recent studies indicate otherwise. Despite their importance, fertility estimates for the house mouse are incomplete because traditional measurements, such as anaphase I nondisjunction and germ cell death, are rarely estimated in conjunction with litter size. In an attempt to bridge this gap, we have taken advantage of the house mouse hybrid zone in Upper Valtellina (Lombardy, Italy) in which five Rb races interbreed. We present data on the fertility of naturally occurring ("wild-caught") hybrids and of offspring from laboratory crosses of wild-caught mice ("laboratory-reared"), using various measurements. Wild-caught mice heterozygous for one fusion were more infertile than predicted from past studies, possibly due to genic hybridity; laboratory-reared heterozygotes carrying seven or eight trivalents at meiosis I and heterozygotes carrying one pentavalent also had low fertilities. These low fertilities are especially significant given the probable occurrence of a reinforcement event in Upper Valtellina.

PMID: 9799266, UI: 99016093

6. An observed chromosome fusion. Hereditas 1998;129(2):177-80 A new centric fusion translocation in cattle: rob (13;19). Molteni L, De Giovanni-Macchi A, Succi G, Cremonesi F, Stacchezzini S, Di Meo GP, Iannuzzi L. Institute of Animal Husbandry, Faculty of Agricultural Science, Milan, Italy..

A new Robertsonian translocation has been found in cattle. A bull from Marchigiana breed (central Italy) was found to be a heterozygous carrier of a centric fusion translocation involving cattle chromosomes 13 and 19 according to RBA-banding and cattle standard nomenclatures. CBC-banding revealed the dicentric nature of this new translocation, underlining the recent origin of this fusion. In fact, both the bull's parents and relatives had normal karyotypes. In vitro fertilization tests were also performed in the bull carrying the new translocation, in two bulls with normal karyotypes (control) and in four other bulls carrying four different translocations.

PMID: 10022084, UI: 99146110

7. Cytogenetics and reproduction of sheep with multiple centric fusions (Robertsonian translocations). Bruere AN, Ellis PM. J Reprod Fertil 1979 Nov;57(2):363-75.

The significance of centric fusions (Robertsonian translocations) in domestic animals, with special reference to sheep, is reviewed. The mating is described of a further 856 ewes with either a normal chromosome number 2n = 54 or carrying one or more of the three different translocations (centric fusions) t1, t2 and t3 in various heterozygous and homozygous arrangements. Rams which were used in the matings were homozygous for one of the translocation chromosomes (2n = 52), double heterozygotes (2n = 52), triple heterozygotes (2n = 51) or were carriers of 4 translocation chromosomes (2n = 50) and 5 translocation chromosomes (2n = 49). A remarkably even distribution of segregation products was recorded in the progeny of all combinations of translocation ewes x translocation rams in those groups in which sufficient animals were available for statistical analysis. Forty-eight chromosomally different groups of animals were mated. Further, the overall fertility of the translocation sheep, measured by conception rate to first service, lambing percentage and number of ewes which did not breed a lamb, was not significantly different from New Zealand national sheep breeding data. In some groups the poorer reproductive performance could be explained by the age structure of the flock and inbreeding depression, which probably affected the performance of some animals. Sheep with progressively decreasing chromosome numbers, due to centric fusion, 2n = 50, 2n = 49 and 2n = 48, are reported. The 2n = 48 category represents a triple homozygous ewe and a triple homozygous ram and is the first report of the viable evolution of such domestic animals. Less than 1% of phenotypically abnormal lambs were recorded in a total of 1995 progeny born over 10 years. It is now considered that there is little or no evidence to suggest that centric fusions in a variety of combinations affect the total productive fitness of domestic sheep. It is suggested that future research should be more actively directed to understanding their genetic significance.

PMID: 513026, UI: 80074806

9. Cytogenetic studies of three equine hybrids. Chandley AC, Short RV, Allen WR. J Reprod Fertil Suppl 1975 Oct;(23):356-70

A detailed investigation of testicular meiosis in a mule, a hinny and a Przewalski horse/domestic horse hybrid were made. Abnormalities of pairing were observed in the mule and hinny in most germ cells at the pachytene stage of meiotic prophase, and spermatogenesis was almost totally arrested. A few mature spermatozoa were recovered from the ejaculate and epididymal flushings of the hinny. The Przewalski horse/domestic horse hybrid was fertile and showed normal spermatogenesis. Chromosome banding studies showed a close homology between the karyotypes of the Prezwalski horse (Equus przewalskii, 2n = 66) and the domestic horse (E. caballus, 2n = 64), and it is evident that a single Robertsonian translocation has occurred transforming four acrocentric chromosomes of E. przewalskii into two metacentric chromosomes in E. caballus. The investigations showed that a trivalent is formed at meiosis in the hybrid (2n = 65), segregation from which gives two classes of genetically balanced spermatozoa. Both of these are capable of producing normal offspring if they fertilize the eggs of a domestic mare.

PMID: 1060807 [PubMed - indexed for MEDLINE]