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Species Concepts and Reproductive Isolating Barriers

Author(s): Tadzia GrandPré, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Hybrid Sterility

In some cases, hybrid zygotes successfully develop into adults but the adult individuals do not produce viable gametes. This isolating barrier, known as hybrid sterility, restricts the amount of genome mixing that can occur across two species.

A classic example of a sterile hybrid is the mule, the offspring of a female horse and a male donkey. All mules typically are sterile. However, within other taxonomic groups, hybrid sterility is displayed in only one sex. For example, in the fly genus Drosophila, crosses between different species produce sterile male hybrids and female hybrids that usually are fertile.

When hybrid sterility affects one sex preferentially, it is generally the heterogametic sex (the sex with two different sex chromosomes) that is sterile, while the homogametic sex (the sex with a matching pair of sex chromosomes) is fertile. This is known as "Haldane's rule."  In most mammals and insects, including flies, the males are heterogametic and the females are homogametic. Birds and reptiles are the opposite. That is, the males are homogametic and the females are heterogametic.

Hybrid sterility is both a postmating and a postzygotic isolating barrier.