Emily Spiegel (’18) and Erin Bonner (’17)
Dr. Greg Davis
Biology Department

Photoperiod Response in Aphid Polyphenism

This summer, in the lab of Dr. Greg Davis, we will be studying the evolution of reproductive plasticity in Acyrthosiphon pisum (pea aphids). Aphids exhibit an exceptional response against the perils of winter. During warm summer months, asexual female aphids reproduce at a rapid rate, quickly overtaking their flora of choice. Yet, as nights grow longer in the fall, heralding the onset of winter and cold temperatures, these asexual females produce sexual females and males. These sexually reproducing individuals mate and lay frost-resistant eggs, which will hatch in the spring, releasing asexual females. The cycle repeats, maximizing reproductive fitness with quick reproduction throughout the warm summer, and ensuring both genetic variation and survival through the winter with diapausing eggs. Previous research suggests that the execution of this switch to sexual reproduction is mediated by juvenile hormone (JH). The current view is that JH acts as an asexually promoting signal during short night conditions (i.e., during summer months), allowing the aphids to maintain asexual reproduction when environmental conditions favor it. The goal of our research this summer is to more critically evaluate the conditions necessary for this switch, as well as investigate the possible pathway of induction of this switch through JH.

Our first aim is to test the photoperiod response of three strains of aphids in order to ascertain whether local adaptation has occurred. This we will do by attempting to identify the Critical Night Length (CNL) of each strain, which we suspect will be different for each strain. We define the CNL as the minimum number of hours of darkness that aphids must be exposed to in order to produce 50% sexual offspring over the first 5 days of larviposition (giving birth). We will test multiple photoperiods in the different strains using a newly developed and more efficient method. We are building a staggered light box, with ten compartments, each of which will undergo a different night length in a 24 hour cycle. Our four strains are from Upstate New York; Tucson, Arizona; and Atlanta, Georgia (2 strains). As a result of differences in the timing and appearance of cold temperatures during winter between these locations, it is believed that the more southern strains have evolved over time to require more darkness to induce sexual reproduction, compared to the more northern New York strain. Although the TUCSON strain is capable of producing sexuals at extreme dark photoperiods, we are testing the hypothesis that this strain does not produce sexuals in its natural habitat. It is unknown whether the Georgia strains are capable of producing sexuals at all, and if so, if they produce sexuals at photoperiods experienced in Atlanta. If we determine that the CNL for these southern strains is longer (i.e. requires more darkness) then we will have found evidence of local adaptation.

Our second aim is to test a possible mechanism behind the potential differences in photoperiod response between the strains. As was discussed earlier, JH has been implicated in the pathway to asexual reproduction in aphids. One hypothesis is that any observed difference in CNL between strains is due to increased sensitivity to JH in the southern strains. To test this hypothesis, kinoprene, a JH analog, will be applied in varying doses to adult females that normally produce sexual progeny and their progeny scored for asexual or sexual fate. Previous research in the Davis Lab has found a possible difference in JH sensitivity between TUCSON and the New York strain (LSR1) using kinoprene. Specifically, TUCSON was found to be more sensitive to JH than the New York strain, producing a greater percentage of asexuals at the same dosage. We will repeat this experiment with greater numbers at finer dosage resolution, and include the two Georgia strains.