A New Discovery: Genetically Linked Sets of Reproductive Traits

In populations of the white-throated sparrow (Zonotrichia albicollis), a common North American songbird, there are two types of individuals: white- and tan-striped (see the figure). Although these types occur in both sexes, here we discuss just the males. Tan-striped males differ from white-striped males in three traits: stripe color, mating behavior, and parental behavior. Males that have brilliant white stripes on their heads tend to be aggressive in acquiring mates and relatively inattentive to their offspring. Tan-striped males, besides being drab, are low in aggressiveness and high in offspring attentiveness. Biologists have long realized that cases like this raise a profound question: Why do certain traits (type of coloration, degree of aggressiveness, and degree of attentiveness in this case) consistently occur together? Why does it seem that, in some cases, reproductive traits evolve as a set—a single complex phenotype—rather than showing independent trait evolution?

One theory (among several that exist) is that genes affecting the traits in a set are linked in the chromosomes. In the last few years, dramatic genomic evidence for this concept has been published on at least two bird species, one being the white-throated sparrow.

Figure A White-throated sparrows occur in two “morphs” that differ in coloration and behavior White– and tan–striped adults display consistent behavioral differences. (Photos from Horton et al. 2014.)

Genomic studies have revealed in these sparrows a very large set of genes (about 1000) that are inherited together as a supergene (a single set) because of being located in a chromosomal inversion. Tan-striped individuals lack this supergene, whereas white-striped ones are heterozygous for it. The genes for all three of the reproductive traits in the set discussed here are believed to be in this supergene, as are genes for steroid-hormone receptors (noteworthy because the behavioral traits are hormone-sensitive). Box Extension 17.1 provides references on this breaking story.

References

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