Roundworm infections cause several debilitating diseases in both animals and agricultural plants, yet relatively little is known about the factors that regulate either the population dynamics and/or infectivity of most parasitic or free-living nematode species. This proposal aims to uncover the mechanisms by which a specific set of closely related roundworm species regulate the ratio of males, females, and/or infective/dispersive larval forms amongst their offspring. A combination of cellular, chemical and genetic approaches will be used. Broader impacts of the proposal studies include establishing research and student training links between a primarily undergraduate institution (William & Mary) and an institution with a large proportion of minority students (UT-Arlington). In addition, these studies may ultimately lead to new avenues for controlling roundworm infections.
Reproduction is a fundamental characteristic of all living creatures. We are interested in elucidating the factors that influence how organisms reproduce, and how different strategies evolve to maximize the passing of genes on to the next generation. The research supported by this award was designed to study how organisms produce different genders, and whether the production of sons or daughters is dependent on genetic and/or environmental factors. We used as model system a small (~1 mm), free-living roundworm with the provisional name SB347, which produces three genders (males, females and selfing hermaphrodites). SB347 shares life-history traits and reproductive strategies that are similar to some parasitic nematodes. For instance, many parasitic nematodes have a free-living stage composed of males and females that cross to generate the infective stage, which is usually the larval form of a female or hermaphrodite. Similarly to these parasites, SB347 also produces almost exclusively females or hermaphrodites after crossing with males. However, it is not understood how so few males are produced after outcrossing. Based on the karyotype of males (XO) and non-males (XX), one would expect about 50% of male progeny. We found that SB347 males discard the nullo-X sperm by the process of programmed cell death. This results in males containing mostly only one kind of functional sperm (X-bearing sperm), which when fertilizing the X-bearing oocyte of the opposite sex generate XX progeny. How is the other decision made, of whether an XX individual develops into a hermaphrodite or female? We found that the environment encountered by the mother or by juveniles is the strongest factor to influence the production of hermaphrodites or females. When exposed to nematode chemicals produced under crowded conditions, XX larvae of SB347 and related nematodes develop mostly into hermaphrodites instead of females. We found that hermaphrodites obligatorily pass through a larval stage named ‘dauer’ that is similar in behavior and physiology to the infective stage of parasitic nematodes. Dauers are adapted to migrate to new environments and to resist to hazardous environmental conditions. This mode of reproduction, in which a self-propagating generation passes through a migrant and resistant stage, is reminiscent of many parasitic nematodes. The results derived from this award are useful not only to understand how evolution has shaped different modes of reproduction in this worm, but also to devise strategies to restrict the spread of roundworm parasites.
