D a w n..H i g g i n s o n
Ph.D. Graduate Student

Academic advisor: Scott Pitnick

Email: dmhiggin@syr.edu
Telephone: 315-443-5128
Office Location: 004 Lyman

EDUCATION:

RESEARCH

Sperm conjugation, where two or more spermatozoa join at the head to form a single functional unit, is one of the most fascinating variations in sperm morphology.  First described more than one hundred years ago, conjugation has evolved independently numerous times and has been reported in a variety of taxa .  Despite the potential impact of conjugation on sperm competition and gamete evolution, it has been the focus of very little research. Conjugation represents an unusual transition from individual sperm to group behavior; members of a conjugate swim in a coordinated manner before dissociating at the site of storage in a female .  The few studies examining this phenomenon indicate that conjugation enhances swimming speed, thus improving the competitive ability of a male's ejaculate and increasing reproductive fitness. Sperm competition is prevalent in most animals , yet conjugation is exceedingly rare .  If cooperation among sperm within an ejaculate can enhance fertilization success when competing against a common enemy (i.e., another male's sperm), why is sperm conjugation the exception instead of the rule in nature?  I am studying this question, and others, as part of my dissertation research.

Conjugation is particularly prevalent in Hydradephaga, a monophyletic group of aquatic beetles.  Although v ery little is known about their reproductive biology , there is some evidence of sexual conflict and sperm competition avoidance (e.g. mating plugs and mate guarding) in Hydradephaga.  T he exceptional adaptations for conjugation, however,  suggest that aquatic beetles have unusual mating systems.  Study of these beetles will offer unique insight on the selective forces acting on sperm morphology and the nature of postcopulatory sexual selection.

Functional Morphology

Female reproductive tracts form the selective environment for sperm.  Dytiscids, the largest family in Hydradephaga, have remarkably complex and divergent reproductive tracts.  I am examining the connection between female and sperm morphology, as well as, the role of conjugation in sperm motility in this family.

There are at least three different mechanisms of conjugation in Hydradephaga, 1) sperm heads are cemented together by extracellular material, 2) the tip of one sperm head slips inside the hooded portion of another (like stacking cups), and 3) sperm heads attach to a rod of unknown origin, called a spermatostyle.  Conjugation occurs after sperm individualization and single sperm aggregate with other spermatozoa lying in close proximity in the vas deferens.   The spermatostyle-mediated conjugation is a possible exception to this ; sperm attached to a rod may be products of a single cyst. Once in a female, spermatozoa dissociate from the spermatostyle leaving naked rods.  I am exploring the spermatogenic origin of the spermatostyle, the location of conjugation and the fate of the rods in females (i.e. dumped, encysted or digested).

PUBLICATIONS

Bjork, A., W.T. Starmer, D.M. Higginson, C.J. Rhodes and S. Pitnick. 2007. Complex interactions with females and rival males limit the evolution of sperm offense and defense. Proceedings of the Royal Society of London B, 274:1779-1788.

Tabashnik, B. E., Biggs, R. W., Higginson, D. M., Henderson, S., Unnithan, D. C., Unnithan, G. C., Ellers-Kirk, C., Sisterson, M. S., Dennehy, T. J., Carrière, Y. and S. Morin.  2005. Association between resistance to Bt cotton and cadherin genotype in pink bollworm.  J. Econ. Entomol. 98(3): 635-644. [pdf]

Higginson, D. M., Morin, S., Tabashnik, B. E. and Y. Carrière.  Evolutionary trade-offs of insect resistance to Bt: fitness cost affecting paternity.  2005.  Evolution.  59(4): 915-920. [pdf]

Carrière, Y., Ellers-Kirk, C., Biggs, R., Higginson, D. M., Dennehy, T. J., and B. E. Tabashnik. 2004.  Effects of gossypol on fitness costs associated with resistance to Bt cotton in pink bollworm.  J. Econ. Entomol. 97(5): 1710-1718. [pdf]

Morin, S., Biggs, R. W., Sisterson, M. S., Shriver, L., Ellers-Kirk, C., Higginson, D., Holley, D., Gahan, L. J., Heckel, D. G., Dennehy, T. J. Brown, J. K. and B. E. Tabashnik.  2003.  Three cadherin alleles associated with resistance to Bacillus thuringiensis in pink bollworm.  Proceedings of the National Academy of Science, USA.  100 (9): 5004-5009. [pdf]

Gillespie, D. R., Higginson, D., Foisy, M., Quiring, D. and R. R. McGregor.  2000. Potential Arthropod Pests of Greenhouse Vegetable Crops in British Columbia – A Review and Threat Assessment. Pacific Agri-Food Research Centre, Agassiz, Technical Report #160.  Agriculture and Agri-Food Canada.

McGregor, R. R., Gillespie, D. R., Quiring, D. M. J. and D. Higginson.  2000.  Parasitism of the eggs of Lygus shulli and Lygus elisus (Heteroptera: Miridae) by Anaphes iole (Hymenoptera: Mymaridae).  Journal of the Entomological Society of British Columbia. 97: 89-93.