Alternative seed defence mechanisms in a palo verde (Fabaceae) hybrid zone: effects on bruchid beetle abundance

Abstract

Abstract. We asked three questions about the patterns of relative abundance of insect herbivores across host plant taxa at a palo verde hybrid zone. (1) What is the morphological structure of the hybrid zone and does this suggest a certain pattern of introgression? (2) How are putative parental seed defence mechanisms expressed in hybrid plants? (3) Do ovipositing females prefer host plant taxa on which their offspring have best survivorship? Morphologically, hybrids were either intermediate or tended to resemble one parental species. Previous studies have suggested that unidirectional introgression results in loss of parental defence mechanisms against herbivores. Hybrid plants in general lacked seed coat resistance and early pod abscission which are known to act as plant defence mechanisms against bruchid beetles in the parental palo verde trees. All other sources of bruchid mortality that we examined did not vary across parental and hybrid taxa, with the possible exception of egg parasitism which occurred at a lower frequency on one parental palo verde species. Thus, survivorship of bruchid offspring should be greater on hybrid palo verdes. Patterns of egg densities suggested that females may prefer hybrid hosts in some years but not others. An oviposition choice experiment conducted in the field, however, showed bruchids have no preference for hybrids over one of the parental species. These results suggest that some insect herbivores may have higher densities on hybrid host plants because they are less resistant.

Differences in production and mortality of reproductive structures in two Prosopis L. (Mimosaceae) shrub species from Patagonia, Argentina

“…The abortion of inflorescences and flowers prior to anthesis precludes both the attraction of pollinators and the possibility of flowers functioning only as pollen donors, because in Prosopis, pollinators do not visit the flowers and pollen is not transported by them, until the moment in which the flower opens completely and nectar appears (Genise et al, 1990(Genise et al, , 1991. Similar patterns of flower-bud abortion, before either pollen donation or pollination is possible, has been described for other perennials by Siemens et al (1994) and Bowers (1996), among others. Ehrle´n (1991) proposed that surplus flowers are an expression of the plant's being selected to maintain enough ovaries to match the resources available for reproduction after predation, non-fertilization, and other mortality causes that are not maternally regulated.…”

“…This point requires further manipulative investigation. The other mechanism, the abortion of flowers within an inflorescence, has been explained in other species as a strategy to avoid the energetic and mechanical costs of maturing several offspring at one site (Stephenson, 1981) or to avoid direct herbivory by insects (Siemens et al, 1994).…”

Effects of plant hybridization on herbivore-parasitoid interactions

“…Preszler and Boecklen (1994) found lower parasitism of Phyllonorycter on oak hybrids compared to parental species. Siemens et al (1994) suggest that parasitism of bruchid weevils on palo verde was lower on one parent species than on hybrids and the other parent, and Gange (1995) found that parasitism of aphids was intermediate on hybrids. The results of these few studies are variable and suggest that there may be as much variation in the tritrophic effects of plant hybridization as there is variation in the direct effects of hybrid plants on the herbivores.…”

“…The effects are highly variable, ranging from higher or lower abundance of herbivores on hybrid plants to intermediate abundance or even no differences between hybrids and parent species (Whitham 1989;Boecklen and Spellenberg 1990;Boecklen and Larson 1994;Fritz et al 1994Fritz et al , 1996Morrow et al 1994;Whitham et al 1994;Christensen et al 1995). Little attention has yet been given to the effects of plant hybridization on herbivore fitness, particularly via their interactions with natural enemies (but see Moorehead et al 1993;Preszler and Boecklen 1994;Siemens et al 1994;Gange 1995). Understanding the tritrophic interactions involving hybrid and parental plants, herbivores, and natural enemies may explain the variable abundance and feeding preferences of herbivores on hybrid and parental plants.…”

Large acorns benefit seedling recruitment by satiating weevil larvae in Quercus aliena

“…Lots of seed-eating animals exert strong selective pressures on plants by greatly reducing seed crops, and thereby lowering plant fitness and fecundity (Steele et al 1993;Siemens et al 1994). Plants, however, evolve a number of morphological and chemical characteristics that serve as defense mechanisms against seed predators and dispersers (Harborne 1991;Steele et al 1993;Hughes and Vogler 2004), in which some plant defenses appear only when the plant is attacked (Howe and Westley 1988;Schultz 1988).…”