Principles of Genetics and Breeding

7.1.3.3 Sex chromosomes and colour patterns

7.1.1.3 Sex chromosomes and colour patterns

According to Traut and Winking (2001), three fish species are representative of the basic steps in sex chromosome evolution:

(1) the zebrafish, Danio rerio, has environmental or polygenic sex determination without heterosomes;

(2) the southern platyfish, Xiphophorus maculatus, which has genetically defined sex chromosomes; and

(3) the guppy, in which the acrocentric Y chromosome can be identified cytogenetically.

• Comparative genomic hybridization (CGH) indicated that a large part of the nonpairing region of the guppy Y chromosome comprises male-specific repetitive DNA.
• Only one-half of the Y chromosome pairs with homologous regions of the X in synaptonemal complexes.
• Orientation of guppy heterosomes allowed the recombination in only two of 49 observed synaptonemal complexes, suggesting that crossover is greatly reduced even in the homologous region (Traut and Winking, 2001).

The color patterns of guppies are complex and often conspicuous combinations of orange, black,white, yellow, green, iridescent blues, occurring as spots, speckles, bars, and lines.

• These color patterns are expressed mostly in males - only patterns that have never been reported from wild populations show weak expression in females.

Lindholm and Breden (2002) reviewed literature on linkage of sexually selected male traits in guppies, and found that

• the inheritance of attractive male traits shows that color patterns, caudal fin size and shape, courtship rates, and general attractiveness are primarily sex linked.
• They also reported that 26 traits are Y-linked, two are X-linked, 24 are X and Y linked, and nine are autosomal.
• Crossing-over makes possible recombination of some traits encoded by sequences located on sex chromosomes.
• However, there is a restricted region of the chromosome where crossing-over cannot take place - the location of sex-determining genes.
• The genes situated close to the sex-determining locus can cross over but seldomly do so.
• Rare crossing-over events at a frequency of < 1/3,800 have occurred between guppy genes for red and black elements in the Maculatus color pattern, which are believed to be located very close to the sex-determining locus (Winge 1934;Basolo 2006).
• Furthermore, a new and complex linkage map for the guppy based on phenotypic traits and genetic markers suggests that the sex determining region is flanked on both sides by recombining regions (Khoo et al, 1999; Khoo et al,2003).
• The extraordinary polymorphism of male guppy color pattern and fin form is maintained in populations and this persistence is perpetuated by evolutionary mechanisms.
• No two male guppies have identical color patterns in the wild even though crossing-over between X and X chromosomes, X and Y chromosomes, and autosomal gene recombination are the most important sources of variability of male guppy color patterns and fin morphology.
  

  Significance

  1. It provides a direct evidence of the linear arrangement of genes in the chromosomes.
  2. Chromosome maps can be constructed.
  3. It gives rise to new combinations of genes, and hence, variations in offspring.