Michael Palopoli et al. (2007) International Worm Meeting "Polymorphism at the Caenorhabditis elegans plg-1 locus is due to a retrotransposon insertion that blocks transcription of a mucin-like protein."

TinMaung Aye, Camden Ramsay, Michael Palopoli, Matthew Rockman, Curwen Stephen, Jason Laurita, Leonid Krugylak

[International Worm Meeting, 2007]

Natural populations of C. elegans are polymorphic for the ability of males to deposit a copulatory plug after mating. Plugs have a mate-guarding function in this species (Barker 1994, Animal Behaviour 48:147), and the polymorphism is due to allelic variation at the plg-1 locus (Hodgkin and Doniach 1997, Genetics 146:149). We hypothesized that the polymorphism is caused by a loss-of-function mutation because (1) plugging is likely to be ancestral, since plugs have been observed in other Caenorhabditis species; (2) the plugging allele is dominant; and (3) males are rare in natural populations, so a mate-guarding function is probably dispensable. To test this hypothesis, we have identified the molecular basis for the plg-1 polymorphism. We narrowed the location of plg-1 to a 76-kb interval using a combination of recombination and physical-mapping experiments. We then discovered that natural populations of C. elegans are polymorphic for the presence of a retrotransposon insertion (Cer-1) within this genomic interval. Interestingly, this retrotransposon is inserted within genomic sequence that is predicted to code for a mucin-like protein. Furthermore, the insertion is present in all of 45 non-plugging isolates, but absent in all of 92 plugging isolates that we tested. RT-PCR experiments showed that the mucin-like gene is transcribed in adult males, but not in adult hermaphrodites, of plugging strains. In contrast, the transcript was not observed in non-plugging strains, suggesting that the retrotransposon blocks transcription of the mucin-like protein. The results of RNAi knockdown and transgenic rescue experiments confirmed the identity of the mucin-like protein as the plg-1 gene. The plg-1 polymorphism is caused by a naturally segregating retrotransposon insertion that disrupts production of a structural component of the copulatory plug.

Michael Palopoli et al. (2008) Development & Evolution Meeting "Molecular Basis of the Copulatory Plug Polymorphism"

Camden Ramsay, Aye TinMaung, Jason Laurita, Leonid Kruglyak, Michael Palopoli, Matthew Rockman, Andrea Aduna, Stephen Curwen

[Development & Evolution Meeting, 2008]

Understanding the genetic basis of phenotypic variation in natural populations remains one of the central problems in modern biology. We investigated the genetic basis of a classic phenotypic dimorphism in 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 plugs. The copulatory plug is a gelatinous mass that covers the hermaphrodite vulva, and it appears to serve a mate-guarding function in this species. We show that variation in plugging ability 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, it occurs primarily as self-fertilizing hermaphrodites. 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.