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

Evolution of the Photoperiod Response in Pea Aphid ReproductivePolyphenism

We are studying the role of photoperiod response in the evolution of Acyrthosiphon pisum (pea aphid) reproductive plasticity. Pea aphids exhibit a remarkable evolutionary response to the challenges posed by seasonal fluctuations in climate. During warm summer months, female aphids reproduce asexually, allowing them to quickly produce vast numbers of genetically identical offspring. However, as nights grow longer in the fall, aphids switch to sexual reproduction. In response to longer night conditions, they are induced to produce sexual females and males that will mate to lay fertilized eggs. These eggs are durable and frost-resistant, allowing the species to survive through the cold winter months. In the spring, the eggs hatch into asexually-reproducing females, and the cycle continues. By alternating their mode of reproduction in response to seasonal fluctuations in night length, pea aphids are able to maximize their reproductive fitness: they reproduce rapidly and efficiently throughout the spring and summer, and by maintaining the ability to reproduce sexually, are able to survive the cold winter and generate beneficial genetic diversity. Previous research in our lab has suggested that Juvenile Hormone (JH) signaling mediates the switch between asexual and sexual reproduction. It is thought that JH sends an asexual-promoting signal during the spring and summer (short night conditions), thereby specifying asexual reproduction during favorable environmental conditions. The goal of our research this summer is to more critically evaluate the conditions that control this switch in reproductive fate, and to further investigate the role of JH in inducing this switch.

We are first testing the photoperiod response of three strains of aphids in order to ascertain whether strains collected from different latitudinal regions have undergone changes in their response to increasing night length. We will do this by attempting to identify the Critical Night Length (CNL) of each strain, in the belief that different strains will have different CNLs. The CNL refers to the minimum hours of darkness that aphids must be exposed to in order to produce 50% sexual offspring over 5 days. We will test multiple different photoperiods in the different strains, using a newly developed and efficient method: we are building a staggered light box with ten compartments, each of which will simulate a different night length in a 24 hour cycle. The three strains being tested are from Tucson, Arizona; Atlanta, Georgia; and upstate New York. The strain from Tucson does not produce sexuals in its natural habitat, because the warmer temperatures and less severe winters mean that they do not need to produce frost-resistant eggs. It is unknown whether the Georgia strain will produce sexuals. As a result of the latitudinal, and thus climatic, dissimilarities between these locations, it is believed that the more southern strains have evolved over time to require more darkness to induce sexual reproduction, in comparison to the more northern New York strain.This is because it would be disadvantageous for the more southern strains to produce sexuals if the winters were not severe enough to require this mode of reproduction. To support this theory, we aim to show that the CNL for more southern strains is longer, i.e. it requires longer exposure to darkness to induce sexual reproduction.

The second part of our research will involve testing a possible mechanism behind the differences in photoperiod response between the strains. As discussed earlier, JH has been implicated as specifying asexual fate in aphids. We postulate that the probable difference in CNL between the strains may be explained by an increased sensitivity to JH in the southern strains. We are applying Kinoprene, a JH analog, in varying doses to adult aphids, and scoring their progeny for asexual or sexual fate. Previous research from this lab has found a difference in JH sensitivity between Tucson and the New York strain using kinoprene. The Tucson strain was found to be more sensitive to JH, producing a greater percentage of asexuals than the New York strain given the same dosage treatment. This experiment needs to be repeated, and we will also be narrowing the dosage treatments and applying these treatments to the Georgia strain as well.