Density-dependent fecundity and infective juvenile production in the entomopathogenic nematode, Heterorhabditis megidis

Abstract

summaryThe entomopathogenic nematode Heterorhabditis megidis may undergo several rounds of reproduction within a single host. Infective juveniles (IJs) are formed within each generation during a process referred to as endotokia matricida, which involves the progressive consumption of the parent hermaphrodite or female by the developing IJs prior to emergence from the host cadaver. The present study examines the extent to which within-host population dynamics exhibit densitydependent variation. Particular attention is paid to the effect of infection density on the relative production of IJs and ' normal ', non-infective offspring within each generation and on the emergence of the IJs from the host. Fecundity was found to be negatively density dependent across generations. However, at high infection density the first generation hermaphrodites invested relatively more in IJs at the expense of producing non-infective offspring. It is suggested that this pattern resulted from an adaptive, phenotypically plastic allocation of reproductive investment between offspring types in response to increased competition. The F " and F # IJs were also shown to emerge from the host in relatively discrete pulses.

Phased infectivity in Heterorhabditis megidis: the effects of infection density in the parental host and filial generation

“…Further details of the experimental infections can be found in Ryder and Griffin (2002). Briefly, three groups of 10 replicate dishes of G. mellonella larvae (nine larvae per dish) were exposed to a dose of either 50, 500 or 1,000 IJs per larva.…”

“…This pattern occurred at each of the three parental infection densities, making it possible to harvest largely pure samples of F 1 and F 2 IJs at all three densities. IJs were harvested on every day of the emergence period for each replicate, but for the infectivity tests carried out in the present study (see below) only IJs collected on those days that corresponded to the peak of emergence for each generation were used (18 -20 days after the initial infections were carried out for the F 1 IJs, and 23-25 days after the initial infection for the F 2 IJs; see Ryder and Griffin, 2002, for details).…”

“…In a previous study, we found that H. megidis IJs (UK211 strain) from the first filial generation (F 1 ) emerged from their test host in a discrete pulse several days before the F 2 IJs emerged (Ryder and Griffin, 2002). This led us to predict that F 1 IJs will show an earlier peak in infectivity following emergence than F 2 IJs.…”

“…Several filial generations usually occur during the course of an infection (Wang and Bedding, 1996); each is comprised of non-infective offspring that are capable of immediate further reproduction, and a variable number of IJs. As the host's resources are depleted, an increasing proportion of the offspring in each generation develop into IJs and emerge in order to seek a new host (Ryder and Griffin, 2002).…”

“…We also tested this prediction for IJs collected from hosts infected with three different densities of parent nematodes. The total number of early-emerging (F 1 ) IJs increases with infection density in the parental host in H. megidis (Ryder and Griffin, 2002) and would be expected to have a strong effect on the adaptive value of delaying infectivity, as it should increase the risk of overcrowding within nearby hosts.…”