Did parasites evolve thanks to arrested development?

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The evolution of complex traits and life history strategies can often involve the process of preadaptation, through which the evolution of a novel trait is facilitated by an existing feature which previously fulfilled a different role. These features can cover a multitude of anatomical, physiological and behavioral adaptations which include physical characteristics as well as intricate mechanisms and pathways.

Stasiuk et al present research in EvoDevo this week surrounding the use of a preadaptive feature in the evolution of parasitism in nematode worms. Nematode parasitism has been speculated to have evolved via a preadaptive mechanism due to the probable high number of evolutionary events leading to parasitism within the group. Furthermore, these events have occurred in species which are interspersed with non-parasitic species, and therefore, all parasitic species in the group cannot be united through a single common parasitic ancestor.

In order to test this theory, and their own hypothesis that the dauer larva (a developmentally arrested morph of many free living nematodes, likely to have been present in the last common ancestor to nematodes) represents a possible preadaptation to parasitism, Stasiuk et al discuss a series of experiments into the facultative development of either parasitic or free living life-cycles. They compare life history strategies of Caenorhabditis elegans, a free living nematode with dauer stage larva, and Parastrongyloides trichosuri, a nemataode which uses both parasitic and free living life history strategies, dependent on environmental cues.

The authors conclude that their experiments do indeed support the hypothesis of the evolution of parasitism in P.trichosuri through a preadapted developmentally arrested life stage similar to that of the dauer larva in C.elegans. They show that similar environmental cues such as temperature, population density and food availability may affect the formation of dauer larva in C.elegans and uptake of a parasitic life mode in P.trichosuri.

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