Inbreeding Alters Resistance to Insect Herbivory and Host Plant Quality in Mimulus Guttatus (Scrophulariaceae)

Abstract

Abstract. Previous studies have demonstrated genetic variation for resistance to insect herbivores and host plant quality. The effect of plant mating system, an important determinant of the distribution of genetic variation, on host plant characteristics has received almost no attention. This study used a controlled greenhouse experiment to examine the effect of self-and cross-pollination in Mimulus guttatus (Scrophulariaceae) on resistance to and host plant quality for the xylem-feeding spittlebug Philaenus spumarius (Homoptera: Cercopidae). Spittlebugs were found to have a negative effect on two important fitness components in M. guttatus, flower production and aboveground biomass. One of two M. guttatus populations examined showed a significant interaction between the pollination and herbivore treatments. In this case, the detrimental effects of herbivores on biomass and flower production were much more pronounced in inbred (self) plants. The presence of spittlebug nymphs increased inbreeding depression by as much as three times. Pollination treatments also had significant effects on important components of herbivore fitness, but these effects were in opposite directions in our two host plant populations. Spittlebug nymphs maturing on self plants emerged as significantly larger adults in one of our host plant populations, indicating that inbreeding increased host plant quality. In our second host plant population, spittlebug nymphs took significantly longer to develop to adulthood on self plants, indicating that inbreeding decreased host plant quality. Taken together these results suggest that the degree of inbreeding in host plant populations can have important and perhaps complex effects on the dynamics of plant-herbivore interactions and on mating-system evolution in the host.

Inbreeding influences herbivory in Cucurbita pepo ssp. texana (Cucurbitaceae)

“…A number of studies have shown that the severity of inbreeding depression varies with environmental stress (Pedersen 1968;McCall et al 1989;Dudash 1990;Schmitt and Ehrhardt 1990;Schmitt and Gamble 1990;Wolfe 1993;Carr and Dudash 1995;Hauser and Loeschcke 1996;Roff 1997;Cheptou et al 2000), and recently herbivory has been shown to exacerbate the severity of inbreeding depression as well (Carr and Eubanks 2002;but see Nunez-Farfan 1996). Therefore inbred plants may both be more likely to be targeted by herbivores and then to suffer disproportionately when exposed.…”

“…Ramsey and Vaughton 1998). Carr and Eubanks (2002) found that the presence of spittlebug nymphs increased the severity of inbreeding depression by a factor of three in one of the two populations that they studied. If inbred plants are less vigorous and grow more slowly than outcrossed plants, they may spend longer in vulnerable life history stages (e.g.…”

“…At the same time more than half of all species self-pollinate 20% or more of the time (Barrett and Eckert 1990), and almost all species show a loss of fitness as a result of inbreeding (Charlesworth and Charlesworth 1987;Husband and Schemske 1996;Crnokrak and Barrett 2002). The effects of herbivory in plant populations are well documented, as are the effects of inbreeding, but little theoretical or empirical work has been done on the interaction between these two ubiquitous forces (Strauss and Karban 1994;Matheson et al 1995;Nunez-Farfan et al 1996;Ouborg et al 2000;Carr and Eubanks 2002). Furthermore, until recently many studies of both herbivory and inbreeding have concentrated on measures of fitness through female function, but even though most species are hermaphrodites and achieve about half of their fitness through male function, relatively little is known about how inbreeding and herbivory affect male function, particularly in concert.…”

Preference for outbred host plants and positive effects of inbreeding on egg survival in a specialist herbivore

“…In plants, inbreeding modifies many resistance traits such as concentrations of secondary compounds [4,5], structural defences [6] and emissions of volatile compounds [7]. Consequently, inbreeding depression in plants is stronger under herbivory, and inbred plant individuals commonly experience greater herbivore damage compared with outbred individuals [3,[8][9][10]. In herbivores, inbreeding reduces for example performance, fecundity and egg hatching rate [5,11].…”

Good mothers, bad mothers, and the nature of resistance to herbivory in Solidago altissima

“…Although we do not have data on the relationship between adult mass and fecundity for spittlebugs, size is generally a very good predictor of fecundity in insects (Wickman and Karlsson 1990;Honek 1993, and references therein). We did not record the sex of the spittlebugs because males and females of this species do not diVer in mass (Carr and Eubanks 2002).…”