Selfish evolution of placental hormones

Abstract

We hypothesize that some placental hormones-specifically those that arise by tandem duplication of genes for maternal hormones-may behave as gestational drivers, selfish genetic elements that encourage the spontaneous abortion of offspring that fail to inherit them. Such drivers are quite simple to evolve, requiring just three things: a decrease in expression or activity of some essential maternal hormone during pregnancy; a compensatory increase in expression or activity of the homologous hormone by the placenta; and genetic linkage between the two effects. Gestational drive may therefore be a common selection pressure experienced by any of the various hormones of mammalian pregnancy that have arisen by tandem gene duplication. We examine the evolution of chorionic gonadotropin in the human lineage in light of this hypothesis. Finally, we postulate that some of the difficulties of human pregnancy may be a consequence of the action of selfish genes.

Keywords: chorionic gonadotropin; gestational drive; infertility; pregnancy; selfish genetic element.

Figure 1.

The gestational drive model of evolution at the LHB and CGB genes. (A) The two haplotypes differ in two ways. The wildtype, d, shows relatively low expression from the placentally expressed gene (CGB) but high expression from the pituitary-expressed gene (LHB). The driving haplotype, D, combines low expression maternally with high expression placentally. (B) Mothers with a dd genotype produce adequate hormone to sustain the corpus luteum through an early stage of pregnancy. (C) Mothers with a Dd genotype produce less hormone than their dd counterparts, and only those offspring who inherit D (maternally derived allele written first) are able to compensate for the reduced maternal expression in order to sustain the corpus luteum through early pregnancy. Offspring of Dd mothers who inherit d cannot produce sufficient hormone to sustain the corpus luteum and are spontaneously aborted.