How to become a parasite without sex chromosomes: a hypothesis for the evolution of Strongyloides spp. and related nematodes

SUMMARYParasitic lifestyles evolved many times independently. Just within the phylum Nematoda animal parasitism must have arisen at least four times. Switching to a parasitic lifestyle is expected to lead to changes in various life history traits including reproductive strategies. Parasitic nematode worms of the genus Strongyloides represent an interesting example to study these processes because they are still capable of forming facultative free-living generations in between parasitic ones. The parasitic generation consists of females only, which reproduce parthenogenetically. The sex in the progeny of the parasitic worms is determined by environmental cues, which control a, presumably ancestral, XX/XO chromosomal sex determining system. In some species the X chromosome is fused with an autosome and one copy of the X-derived sequences is removed by sex-specific chromatin diminution in males. Here I propose a hypothesis for how today's Strongyloides sp. might have evolved from a sexual free-living ancestor through dauer larvae forming free-living and facultative parasitic intermediate stages.

Parasitic Phase of the Sea Lamprey (Petromyzon marinus) in Lake Ontario

Sea lamprey produced in other areas of Lake Ontario appeared to be responsible for significant levels of predation on target species of the eastern outlet basin. The life history of the lamprey is simple with only one parasitic generation present in the lake during the feeding period. Wound frequencies on gillnetted whitefish were influenced by season, fish size, gillnet set duration, and a large random error component which suggested a contagious distribution. Scar frequencies were influenced by fish age and indicated improved survival of whitefish when fish weight exceeded lamprey weight by 43 times. Lamprey impact on the whitefish stocks would probably have been more important at lower fishing intensities. The lamprey may have been prey limited, and size and species preference were probably such that lake trout and burbot were not buffered against sea lamprey by white suckers or whitefish. The analysis favored the view that lamprey were innocuous in 19th century Lake Ontario by reason of prey size and density, but climatic and other environmental effects could also have been important.Key words: sea lamprey, lake whitefish, Lake Ontario

THE MORPHOLOGY AND DEVELOPMENT OF THE SHEEP NEMATODE, STRONGYLOIDES PAPILLOSUS (WEDL, 1856)

In studying the morphology and development of Strongyloides papillosus it has been shown that four molts occur during the development of the free-living sexual adults, two molts in the formation of the infective larvae, and two molts in the development of the parasitic adults from the infective larvae. The last two occur in the body of the host, one in the lungs and the other in the intestine. The time required by the eggs to hatch and the larvae to develop to either the infective larvae or the free-living adults varies with temperature; at 27 °C. it is 6 hr. and 28 hr., respectively. The so-called "spears" in the oesophagus of the free-living adults were found to be the cuticularized tubular endings of the rays of the oesophageal lumen. The head in the free-living adults bears two well developed lips and four papillae, while in parasitic adults it has four lips and four papillae. The tail of the free-living male bears two pairs of preanal and two pairs of postanal papillae. No males were found in the parasitic generation, while parasitic females were found only in the intestine of experimental animals.