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Abstract:
The nematode Strongyloides papillosus is a common intestinal parasite of ruminants. The parthenogenetic, parasitic female produces eggs that either develop directly into infective larvae that are all female or into the females and males of a facultative free-living generation. Different species in the genus Strongyloides differ in their chromosome number and sex determining systems. Some have two autosomes and one X-chromosome (2N=6) and employ an XX/X0-system for sex determination while for others only two chromosomes and no caryotypic difference between the sexes have been described. The later has been proposed to represent an evolutionary loss of a sex chromosome. S. papillosus appears to be a special, probably intermediate, case in that females have two pairs of chromosomes, while males have five chromosomes as a result of a sex specific chromatin diminution event that eliminates an internal portion of one chromosome. Based on cytological studies it was proposed for several Strongyloides species, including S. papillosus, that the males do not contribute genetic material to the offspring but are merely required to initiate parthenogentic development of the egg. It is difficult to imagine that this pseudogamy with the cost of maintaining males but without the advantages of sexual recombination should be evolutionary stable. By using molecular genetic markers we could show that S. papillosus males do contribute genetically to the progeny in most if not all cases, as it had already been shown for S. ratti. Interestingly, some markers are passed on to the offspring in a manner that is consistent with standard, mendelian, autosomal inheritance while for others males pass on preferentially or exclusively only one of their alleles. We are testing the hypothesis that this is the consequence of the particular sex determining system of S. papillosus.
