Bio 205 2014
40 Million Years of Friendship between the Yucca Plants and Their Moths
Yucca plants (genus Yucca) and yucca moths (family Proxidae) are the classic example of desert dwelling species that rely on obligate mutualism to sustain each other. Using the mitochondrial DNA to do a phylogenetic reconstruction of the evolutionary history, they have been able to show that this unique relationship has been developing for an estimated 40 million years. This has led to a very complex relationship between the many species of Yuccasella and Proxidae. (Pellmyr, 9178) While there are an estimated 49 species of plants in the United States alone, each plant can have one or more ...view middle of the document...
While the majority butterflies and moths have a well develop sucking proboscis and gut for ingesting nectar from various plants, Proxidae have neither (Powell, 12). Once fully matured, they do not eat causing a relatively short life as mature moths of 2-4 days. This is not to say that there is no feeding during their entire lifecycle. The species of moth that use locule-oviposting use a long ovipositor to cut through the flowers ovary wall and lay eggs next to the ovules so the larvae has direct access to the developing yucca seeds. The other egg laying habit exhibited is ovipostion just below the perianth tissue surface, leaving the larvae to chew through the tissue to eat a small portion of developing seeds. (Althoff, et al. 399) Once ready, they drop to the ground and burrow to complete the pupa stage of development. When they emerge from the cocoons they crawl back up to the flower where they mate, and begin the cycle all over again. (Powell, 12)
The Proxidae a very rarely oviposit more than one egg per plant ovule, thus regulating the number of moth larvae that can feed on the seeds. (Althoff, 302) Jerry Powell hypothesized that this is because the female Proxidae leave a marking pheromone to avoid overexploitation of the host plant, leading to a single species of moth employing multiple host plants. (13) This behavior, along with the protogynous characteristic of the plant, prevents self-pollination, and passive pollination from other insects. Nearly 100% of Yucca seeds are capable of producing viable offspring. Nourishing the moths allows them to ensure the survival of their species at a small cost of developing seeds. (Thompson, et al. 11487)
As with all mutualistic relationships there arises parasitic, “cheater”, species. There are two hypothesized models for the evolution of the cheaters however the only one backed by evidence is the...