Symbiosis and Mode of Transfer between Hosts

Intimately interacting organisms may have relationships spanning a continuum from pathogenic parasitism to commensalism (in which the participants have no effect on each other) to mutualism (where both benefit species benefit from the association). When a relationship is particularly close so that the symbiont depends on a single host for a significant part of its life cycle, the nature of the relationship might be expected to vary depending on the mode of transmission between hosts. For example, a symbiont that is only transmitted vertically (from parent to offspring) essentially has its reproductive interests synonymized with its host. In this case, selection would be expected to favor the more benign symbionts; symbionts providing a mutualistic service to the host would increase. At the other extreme, if there are frequent opportunities for a symbiont to transfer horizontally (move among unrelated hosts) between hosts, there would be reduced selection against symbionts that harm their hosts. This is particularly true if the relatively harmful symbionts have increased rates of transmission.

Winterschmidtiid Mites and Eumenine Wasps

A number of solitary eumenine wasps (see natural history of eumenine wasps) have symbiotic relationships with mites in the family Winterschmidtiidae. It appears that the relationship between a wasp species and a mite species is quite specific; one species of mite associates only with one species of wasp. The mites and wasps are intimately associated during all life stages. An immature, non-feeding, phoretic stage of the mite (the deutonymph or hypopus) rides on the adult wasp. In most cases the position at which the deutonymphs attach to the host wasp is quite specific and often seems to be a sheltered place that is called the acarinarium (mite chamber). Deutonymphs of the mite Kennethiella trisetosa attach to surfaces between ridges at the side and back of the propodeum ("thorax") of their host Ancistrocerus antilope or inside the genital chamber. Species of the mite genus Vespacarus are associated with the wasp genus Parancistrocerus and cluster in a chamber under the first abdominal tergite.

Mites are transmitted vertically and horizontally. Vertical transmission happens when a female wasp lays an egg in a nest cavity, and a few mite deutonymphs disembark with the egg. They quickly molt to feeding tritonymphs that crawl among the caterpillars that have been provisioned by the mother wasp (see mite development within a wasp cell). Horizontal transmission of K. trisetosa happens during mating (see venereal transmission of mites).

A male A. antilope wasp with a large cluster of K. trisetosa deutonymphs clustered on the right side of the propodeum and thorax (left) and on the back of the propodeum (right).

	Scanning electron micrographs of K. trisetosa on A. antilope. (images courtesy of R. Eversol and the WMU Imaging Center) Click on images for a closer look.

SEM photo of Vespacaris mites peeking from under the first tergite of a wasp in the genus Parancistrocerus. (photo courtesy of J. Cowan)