1. Department of Biology, Bowdoin College, 6500 College Station, Brunswick, Maine 04011, USA
2. Lewis-Sigler Institute for Integrative Genomics and Department of Ecology and Evolutionary Biology, Carl Icahn Laboratory, Princeton University, Princeton, New Jersey 08544, USA
3. These authors contributed equally to this work.

Correspondence to: Michael F. Palopoli^1,3 Correspondence and requests for materials should be addressed to M.F.P. (Email: mpalopol@bowdoin.edu).

Abstract

Heritable variation is the raw material for evolutionary change, and understanding its genetic basis is one of the central problems in modern biology. We investigated the genetic basis of a classic phenotypic dimorphism in the nematode Caenorhabditis elegans. Males from many natural isolates deposit a copulatory plug after mating, whereas males from other natural isolates?including the standard wild-type strain (N2 Bristol) that is used in most research laboratories?do not deposit plugs¹. The copulatory plug is a gelatinous mass that covers the hermaphrodite vulva, and its deposition decreases the mating success of subsequent males². We show that the plugging polymorphism results from the insertion of a retrotransposon into an exon of a novel mucin-like gene, plg-1, whose product is a major structural component of the copulatory plug. The gene is expressed in a subset of secretory cells of the male somatic gonad, and its loss has no evident effects beyond the loss of male mate-guarding. Although C. elegans descends from an obligate-outcrossing, male?female ancestor^{3, 4}, it occurs primarily as self-fertilizing hermaphrodites^{5, 6, 7}. The reduced selection on male?male competition associated with the origin of hermaphroditism may have permitted the global spread of a loss-of-function mutation with restricted pleiotropy.

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