title: Natural variation of outcrossing in the hermaphroditic nematode Pristionchus pacificus.
abstract: BACKGROUND
  Evolution of selfing can be associated with an increase in fixation of deleterious mutations, which in certain conditions can lead to species extinction. In nematodes, a few species evolved self-fertilization independently, making them excellent model systems to study the evolutionary consequences of this type of mating system.
  
  
  RESULTS
  Here we determine various parameters that influence outcrossing in the hermaphroditic nematode Pristionchus pacificus and compare them to the better known Caenorhabditis elegans. These nematode species are distinct in terms of genetic diversity, which could be explained by differences in outcrossing rates. We find that, similarly to C. elegans, P. pacificus males are generated at low frequencies from self-fertilizing hermaphrodites and are relatively poor mating partners. Furthermore, crosses between different isolates reveal that hybrids have lower brood sizes than the pure strains, which is a sign of outbreeding depression. In contrast to C. elegans, P. pacificus has lower brood sizes and the male X-bearing sperm is able to outcompete the X-nullo sperm.
  
  
  CONCLUSIONS
  The results indicate that there is no evidence of any selection acting very strongly on P. pacificus males.
authors.affiliation: Biology Department, University of Texas at Arlington, Arlington, Texas 76019, USA. arielle.click@gmail.com
authors.forename: Arielle
authors.initials: A
authors.lastname: Click
authors.forename: Chandni H
authors.initials: CH
authors.lastname: Savaliya
authors.forename: Simone
authors.initials: S
authors.lastname: Kienle
authors.forename: Matthias
authors.initials: M
authors.lastname: Herrmann
authors.forename: Andre
authors.initials: A
authors.lastname: Pires-daSilva
mesh_terms: D000818:Animals; D012734:Disorders of Sex Development / Q000235:genetics; D019143:Evolution, Molecular; D014644:Genetic Variation; D007178:Inbreeding; D008297:Male; D009348:Nematoda / Q000145:classification / Q000235:genetics* / Q000502:physiology; D009630:Nondisjunction, Genetic / Q000235:genetics; D012641:Selection, Genetic; D012726:Sexual Behavior, Animal; D013094:Spermatozoa / Q000378:metabolism; D013696:Temperature; D014960:X Chromosome / Q000235:genetics