Crossing over in the Sex-chromosome / Menyeberang dalam kromosom Sex

Menyeberang dalam kromosom Sex

Crossing-over in the Sex-chromosome of the Male Budgerigar
[Melopsittacus undulatus]

By: Inte Onsman, Research coordinator

MUTAVI

Research & Advice Group, The Netherlands

Studying sex-linked varieties is extremely interesting especially when one recognizes how many times a crossing-over appears in the offspring. Crossing-overs also show how to learn and understand genetical "laws" in our aviary. In this study crossing-over is discussed, a phenomenon often overlooked by fanciers although it happens all the time. 

The phenomenon of crossing-over in Budgerigars was already discussed by Dr. Taylor and Cyril Warner in 1961. By analyzing results from normal/cinnamon-opaline x cinnamon-opaline matings, involving 53 birds and counting crossovers of both sexes, and by adding the results from normal/cinnamon-opaline x normal matings, involving 29 birds, counting hens only, they obtained the following results: 26 crossovers in a total of 82 birds. The ratio of crossovers to the total number of offspring is known as the crossover value (c.o.v.) which in this case is 26 to 82. Expressing this as a fraction we have 26/82, that is 31.7%, roughly 1:3.

Taylor wrote in his book that slates almost died out during World War II and that the ones that survived were all opaline-slates and most of the present-day slates (he was referring to the early sixties) are opaline in consequence, for he believed the linkage between the two genes appeared to be very close.

In 1970 a friend of mine imported two Budgerigars from England, a skyblue/slate cock and an opaline-slate hen. The birds were bred by the late Cyril Rogers, a well known fancier in England who wrote many books about Budgerigars. My friend was not very successful in breeding with these birds but at last he managed to get some offspring. Unfortunately a few years later he was killed in a car accident and most of the birds he had were lost in an auction. I managed to get hold of one slate and it appeared to be a normal slate cock instead of an opaline. I bred successfully with it for one season. This bird produced a few slate hens and also a few opaline-slate hens, so the slate cock turned out to be split for opaline.

With the results of Taylor in mind I started to breed intensively with slates and I decided to do the same Dr. Taylor had done in the late fifties. Just breeding Budgerigars for exhibitions never gave me satisfaction, I think it is more interesting to discover all the secrets of the Budgerigar.

After having bred slates for more than ten years now it is interesting to make up a balance and see if Dr. Taylor was right about the two genes being closely linked. In the table below the results are present so far and I think they are quite interesting. In order to keep it simple I counted hens only.

Parents :

normal/op-sl [Type I] cock x normal hen

Expected hens

50% op-sl
50% normal

Total amount hens

16

Normal hens

2

Opaline slate hens

4

Crossing overs

3 slate hens

7 opaline hens

Parents

normal/op-sl [Type II] cock x normal hen

Expected hens

50% opaline

50% slate

Total amount hens

38

Slate hens

17

Opaline hens

9

Crossing overs

7 normal hens

5 opaline slate hens

Parentsexpected henstotal amount hensnormal hensopaline slate henscrossing oversnormal/op-sl [Type I] cockxnormal hen 50% op-sl50% normal 16 2 4 3 slate hens7 opaline hens Parents expected hens total amount hens slate hens opaline hens crossing overs normal/op-sl [Type II] cockxnormal hen 50% opaline50% slate 38 17 9 7 normal hens5 opaline slate hens

(In a type I cock sl and op are linked on one chromosome, and in a type II cock sl and op are not linked on one chromosome)

Up until now 54 hens hatched from which 22 were crossovers making the c.o.v. 22 to 54. Expressing this as a fraction we have 22/54, which is 40.5%. These results suggest that Dr.Taylor probably was not right about the close linkage of these two genes. 

The Z-chromosomeThe Z-chromosome has two arms, different in length and connected with each other by a bridge called the centromere. The ratio of these arms is 1:1.5 in Budgerigars, meaning the long arm is 1.5x longer than the short one (Rothfels 1963). The Z-chromosome is the fifth one in line size wise and as long as the fourth one; the fourth one is metacentric, the fifth one is acrocentric, meaning the position of the centromere is not the same. The Z-chromosome contains about 10% (haploid) of the total amount of genes in the Budgerigar. The chance on recombination (c.o.) is not equal at every position along the chromosome, the change is probably smaller close to the centromere and close at the ends than in the middle of the arms of the chromosome. The coincidence on getting a crossover is dependend on the distance between the genes involved. If the distance between two loci (location of genes) is very large, especially on the long chromosomes, sometimes double crossovers are recognized in the offspring.

In birds crossovers between the Z-chromosomes only occur in cocks during meioses. Hens only have one Z-chromosome and crossovers between the Z and W chromosome are impossible because of the heterochromatic character of these two chromosomes.

The point where chromatids cross is called a chiasm after the greek character X. The exchange of genetic information (c.o.) takes place when chromatids form a chiasm, break at the same position and reconnect with the "wrong" ends. More than one chiasm in one arm of a chromosome never leads to a crossover percentage higher than 50. Highly developed chromosomes, such as avian chromosomes, do have a positive interference which means there is only a small chance of finding the points of exchange closely together. This statement suggests that the occurrence of two chiasms on the short arm of the Z-chromosome in Budgerigars would be a rare event.

Double crossing-over frequently occurs in the sex-chromosome of the fowl (Warren 1927) and I thought I recognized several double crossovers in the Z-chromosome of the Budgerigar as well but the result of the next mating yet can be explained by single crossovers although the outcome seems rather complicated;normal/sl-(cin-op) cock x opaline slate hen, as a formula: Zsl / Zcin_op x Zsl_op/ W. This mating produced a total of 23 youngsters, 11 cocks and 12 hens. The outcome of such mating directly shows what happens with the Z-chromosome during meioses. The amount of different phenotypes produced by this mating, appeared to be twice as high as expected; six instead of three, divided as follows:

Normal expectation:Six opaline cocksFive cinnamon-opaline hensTwo slate cocksThree slate hens

Not expected (c.o.):Four cinnamon hensTwo normal cocks (non slates!)One opaline-slate cock

Eventually there are even nine different phenotypes to be expected out of this mating:

1.) Opaline cocks and hens2.) Cinnamon-opaline hens3.) Slate cocks and hens4.) Cinnamon hens5.) Normal cocks6.) Slate-opaline cocks7.) Cinnamon-slate hens8.) Cinnamon-slate-opaline hens9.) Normal hens

The crossover ratio between opaline and lacewing, which I also examined, is also approximately 1:3 (unpublished results). It is very easy to breed opaline-lacewings in the same way described above and also opaline-ino's are not exceptional which is proved by all the opaline cocks being split for ino.

Summary

Testmatings between sex-linked varieties show how mutant genes map on the Z-chromosome. The crossover value between opaline and slate is ± 40.5%. The cinnamon and ino genes are closer linked to slate than to opaline. The c.o.v. between cinnamon and opaline is ± 32% (Taylor 1961) this is in accordance to my findings. The c.o.v. for opaline and lacewing is about the same as for cinnamon and opaline but more test matings are needed. The c.o.v. between cinnamon and slate is still under investigation but first testmatings showed that this must be very low, approximately 5% or less. I hope to report on this within a few years.

Consulted and cited Literature:

[1] Beçak W., Luiza Beçak M, Nazareth H.R.S. and Ohno S. (1964) Close Karyological Kinship between the Reptilian Suborder Serpentes and the Class Aves Chromosoma 15: p.p.606-617 [2] Cole L.J. and Kelly F.J. (1919) Studies on Inheritance in Pigeons, Description and Linkage Relations of two Sex-Linked Characters. Genetics 4: p.p.183-203 [3] Mc Arthur J.W. (1932) Sex-Linked Genes in the Fowl. Genetics 18: p.p.210-220 [4] Owen A.R.G. (1950) The Theory of Genetical Recombination. Advances in Genetics 3: p.p.117-157 [5] Rothfels K.,Aspden M and Mollison M. (1963) The W-chromosome of the Budgerigar, Melopsittacus undulatus. Chromosoma 14: p.p.459-474 [6] Taylor T.G. and Warner C. Genetics for Budgerigar Breeders. London/Illife books Ltd (1961), (1986) [7] Warren D.C. (1927) Sex-Linked Characters of Poultry. Genetics 13: p.p.421-433

©Inte Onsman

MUTAVI Research & Advice Group