zaterdag, juli 19, 2014, 07:01 AM - Russische
door Martin Braak
door Martin Braak
Haldane's rule represents a remarkable observation in the early stage of speciation. The rule also applies to two species which, after allopatric speciation has occurred, form hybrids when secondary contact in an area of sympatry results in incomplete reproductive isolation. It was formulated in 1922 by the British evolutionary biologist J.B.S. Haldane:
"When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex [heterogametic sex]."
Haldane's Rule states that in animal species whose gender is determined by sex chromosomes, when in the first cross offspring of two different animal species, one of the sexes is absent, rare or sterile, that sex is the heterogametic sex. The "heterogametic sex" is the one with two different sex chromosomes (e.g. X and Y); usually the male. The "homogametic sex" has two copies of one type of sex chromosome (e.g. X and X) and is usually the female.
Haldane's Rule for Hybrid Sterility states that a race of animals could diverge enough to be considered separate species, but could still mate to produce healthy hybrid offspring in a normal ratio of males and females. If any of the hybrid offspring were sterile, the sterile offspring would be the heterogametic offspring (males). If the heterogametic offspring was fertile, it produced the normal 50:50 ratio of X and Y sperm.
Haldane's Rule for Hybrid Inviability states that if the divergence between the species became large enough to generate genic differences, but not to prevent mating, then parental gene products may fail to co-operate during development of the embryo, resulting in hybrid inviability (the hybrids are aborted, stillborn or don't survive to maturity). In this case, the male to female ratio of hybrid offspring is skewed with more homogametic offspring while the heterogametic offspring (males) are absent or rare.
Haldane considered the speciation process (i.e the "growing apart" of one species into two species) to occur in stages. The first stage of the speciation process was complete if the two species could mate and produce healthy but sterile hybrids. As the species continued to diverge, they became genetically less compatible. These incompatibilities prevented hybrids from being formed or caused them to die before maturity i.e. it didn't matter whether or not they were sterile since they would not survive to breeding age. These are called "post-zygotic barriers" because a zygote (fertilized egg) is formed, but the offspring (particularly the males) do not breed.
As species differentiation progresses even further, it results in anatomical (body shape), physiological (body function e.g. mismatched pregnancy periods) or psychological (behavioural) differences which prevent the two species mating with each other. Haldane called these "pre-zygotic barriers" because they prevent offspring from being conceived in the first place.
Speciation can involve big jumps as well as gradual shifts and fertile hybrids are more common than Haldane could have realised. The following examples show that some pre-zygotic barriers can be overcome and that there are intermediate stages in post-zygotic barriers. The species involved may have been kept separate by other means e.g. physical separation.
Male Jackals only mate with domestic bitches if the Jackal pups are raised by a domestic bitch (to become imprinted on dogs). There is a psychological barrier, but the offspring are fertile (pre-zygotic barrier, but no post-zygotic barrier). Lions and Tigers must overcome behavioural (courtship) barriers, but produce fertile female offspring and sterile male offspring (pre-zygotic and post-zygotic barriers). Lions and leopards have some physical barriers (size), but these are overcome if the lioness lies on her side to let the leopard mount her; the male Leopons are sterile, though female offspring are fertile (pre-zygotic and post-zygotic barriers). In these cases, pre-zygotic barriers are overcome by rearing the two species together (in whales and dolphins this occurs naturally).
Some cases seem to need additional rules! In Beefalo, Domestic cows may have an immune response against Bison/Cow hybrid calves - this is a physiological barrier, but does not prevent conception. Bison cows don't have this immune response against hybrid calves and hybrid Beefalo males can be fertile. In some hybrids of domestic cats with small wildcats, a proportion of hybrid males are claimed to be partially fertile (incomplete post-zygotic barrier?) and though the hybrid females are fertile they may not successfully raise their young - a psychological barrier, but one which does not prevent mating/conception.
In addition to Haldane's Rules, the viability and fertility of hybrid offspring can depend on which species is the male parent and which is the female parent since some embryo developmental effects come into play depending on which genes come from which parent (e.g. giantism in ligers, but not in tigons).
De "Haldana's rule" is een regel m.b.t. het kruisen van twee soorten en dat hybride mannen vaak onvruchtbaar zullen zijn. Dit in tegenstelling tot de hybride vrouwen. De verklaring hiervoor is de X-chromosoom waarvan mannen één examplaar van hebben en vrouwen twee. Op de X-crhomosoom liggen genen die een wisselwerking aan gaan met een autosomaal gen om een man vruchtbaar te maken. Bij een hybride man kan het X-chromosoom die wisselwerking niet aangaan. Omdat het autosomale gen anders is. Bij vrouwen treedt dit niet op. Voor succesvolle hybridisatie is volgens deze regel nodig dat het X-chromosoom niet wijzigd. Omdat anders geen vruchtbare mannen kunnen ontstaan. Wanneer je bijvoorbeeld het X-chromosoom van de chimpansee vergelijkt met die van de mens, dan zie je veel overeenkomsten.
M.b.t. de Campbelli dwerghamster en de Russische dwerghamster blijkt de Campbelli dwerghamster twee soorten X-chromosomen te hebben waarvan één erg lijkt op het X-chromosoom van de Russische dwerghamster. De F1 hybride dwerghamsters zijn meestal onvruchtbaar. Daar lijkt de Haldana's rule van toepassing!