Maho Okumura

Mentor: Dr. Gregory Davis

Biology Department

 

Juvenile Hormone and the Specification of Reproductive Fate in Pea Aphids


I propose to study the reproductive polyphenism of pea aphids, in which differences in day length determine whether mothers produce daughters that are either sexual or asexual. I have three major goals.

 

First, I plan to examine the role of juvenile hormone (JH) in the reproductive polyphenism of the pea aphid. It has been reported that when JH is topically applied to mothers, she will produce asexual daughters even when she has been exposed to long nights and should produce sexual daughters. Previous students in the same lab have tested the necessity, but not the sufficiency, of JH in specifying asexual fate, using the plant-derived compound, precocene, which destroys cells that actively produce JH. I will repeat this experiment and in particular, I will look at the reproductive fate of aphids that have been treated with precocene as embryos and hence have lost the ability to make JH.

 

Second, I plan to study the differences in the production of offspring. Aphids are known to asexually produce offspring in shorter nights and warmer temperatures (summer), and sexually produce offspring in longer nights and cooler temperatures (winter). Sexual aphids produce eggs, which is a way to survive winter, thus, sexual induction is a way for aphids to survive the winter. Because of this fact, we might expect sexual response to be lost in aphids in milder climate. Strains from different regions of the United States, for example, New York and Arizona, have been reported to respond at different photoperiods, or to not respond at all (i.e., only produce asexual daughters) because strains from lower latitude have adapted to the milder climate. To test this hypothesis, I will use the strain from Tucson, Arizona to see if they produce sexuals at the same length of night as the longest night in their natural habitat (Tucson’s winter solstice). Later, when I am able to obtain strains from Georgia, I plan to perform the same experiment to see if a strain from a latitude similar to Arizona’s produces similar results.

Third, if my second project is successful—that is, if I detect a clear difference in the photoperiod that determines the reproductive fate of the LSR1 strain from New York and the Tucson strain—I intend to perform the JH experiment described above on the Tucson strain. My goal is to test the hypothesis that there are evolved differences in sensitivity to JH between the two strains, in particular, that the Tucson strain is more sensitive to JH than the LSR1 strain. I plan to give the same dosage series of JH to both the Tucson and LSR1 strains to see at what threshold the strains would start producing sexuals. If the Tucson strain has higher sensitivity, it should start producing sexuals at a lower dose of JH compared to the LSR1 strain.