Haldane's rule

Human male karyotpe high resolution - XY chromosome cropped

Haldane's rule is a principle in genetics and evolutionary biology that pertains to the patterns of sterility and fertility observed in hybrid offspring of different species. Formulated by the British geneticist J.B.S. Haldane in 1922, the rule states that if in the cross between two different species or subspecies, one sex is absent, rare, or sterile, that sex is usually the heterogametic sex (the sex with two different sex chromosomes, such as XY in many animals).

Overview

Haldane's rule has been observed in a wide range of animal groups, including mammals, birds, reptiles, and insects. It is considered a cornerstone of speciation research, providing insights into the genetic and evolutionary mechanisms that drive the formation of new species. The rule suggests that genetic incompatibilities are more likely to affect the heterogametic sex because they have only one copy of each sex chromosome, which leaves them more exposed to recessive deleterious alleles.

Mechanisms

Several mechanisms have been proposed to explain Haldane's rule, including:

• Dominance Theory: Suggests that recessive deleterious alleles on the X chromosome can be expressed in the heterogametic sex, leading to sterility or inviability.
• Faster-Male Theory: Proposes that sexual selection drives faster evolution in male-specific genes, which can lead to incompatibilities in the heterogametic sex in hybrids.
• Dobzhansky-Muller Incompatibilities: These are genetic incompatibilities that arise when alleles at two or more loci interact poorly in the hybrid genome, often affecting the heterogametic sex more severely.

Implications

Haldane's rule has significant implications for the study of speciation, the process by which new species arise. It suggests that genetic differences accumulating between species can lead to reproductive barriers, contributing to the formation of new species. The rule also has applications in conservation biology, particularly in understanding the genetic risks associated with hybridization between endangered species and closely related species or subspecies.

Examples

One of the most famous examples of Haldane's rule is observed in the hybrid offspring of a female horse and a male donkey - a mule. Mules are typically sterile and represent the heterogametic sex (XY) in this cross. Another example can be found in the hybrid offspring of female lions and male tigers, known as ligers, where male ligers are often sterile.

Controversies and Limitations

While Haldane's rule is widely accepted and has been empirically supported by numerous studies, there are exceptions and limitations to its applicability. Some hybrids do not conform to the rule, and in some cases, the homogametic sex (XX) may also exhibit reduced fertility or viability. These exceptions suggest that the mechanisms underlying Haldane's rule may be more complex and varied than initially thought.

Conclusion

Haldane's rule highlights the intricate relationship between genetics, evolution, and speciation. It underscores the role of sex chromosomes in reproductive isolation and the evolutionary process. Despite its exceptions, Haldane's rule remains a fundamental principle in evolutionary biology, guiding research in genetics, speciation, and conservation.