Julie Havens
PhD
Candidate
University
of Arkansas
Department
of Biological Sciences
email:
havens@uark.edu
Advisor:
Dr. William Etges
Curriculum
Vitae
Research Interests:
Research in our lab
focuses on
ecological genetics, evolution and speciation. Currently, I
am investigating sexual selection in the desert fly Drosophila
mojavensis;
a species
endemic to the Sonoran Desert and
adjacent arid
lands. Geographically
isolated by the Gulf of California, allopatric populations
of this species differ morphologically and cytologically (Johnson,
1980; Etges et al. 1999), and show differences in
host cactus preferences (Heed, 1982) and life history traits (Etges,
1990; 1993).
Cytological evidence suggests that Baja populations are ancestral to
mainland populations, and that derived populations colonized the
mainland by switching host plants (Heed, 1978; Etges,
1990; 1993).
In the lab, premating isolation has long been
observed (Zouros
& D’Entremont, 1980; Markow et al.
1983; Koepfer 1987a, 1987b; Krebs & Markow 1989; Markow, 1991). Thus,
allopatric populations are considered to
be incipient species. Recently, I have been investigating how
mate choice differs among populations, and how it is influenced by
environment. D.mojavensis
uses more than one host cactus for feeding and breeding, making it an
ideal system with which to investigate environmental influences
on mate choice. Ancestral populations from Baja use
primarily pitaya agria, Stenocereus
gummoses,
while mainland populations have colonized organ pipe cactus, S.
thurberi.
One of the main hypotheses proposed to explain the
premating isolation observed between allopatric populations is that
mate choice patterns have changed as a consequence of adaptation to
different host plants. Mate choice has been
investigated between allopatric populations, in an effort to uncover
the traits involved in sexual isolation. Within population
mate
choice, however, has received considerably less attention.
To investigate sexual
selection, I manipulated the opportunity for mate choice within 2
populations reared on 2 different host cacti. The study was
designed to both allow for and deprive the opportunity for mate choice,
and to measure 2 ecologically relevant components of fitness;
viability and egg-to-adult development time in the offspring.
I found that the opportunity for mate choice led to
significantly decreased offspring development time. This pattern was
population and environment specific, with mainland flies reared on
organ pipe cactus showing the greatest difference due to mate choice
treatment. These results suggest that mate choice confers an
indirect genetic fitness to females, consistent with the 'good genes'
mechanism of sexual selection. Many studies of
sexual selection have not incorporated the effects of
the environment. The results of this study suggest
that 'good genes' sexual selection may not be a general process, and
that a complete understanding of this mechanism will require direct
investigation of environmental influences.
Mate choice in D.
mojavensis is
mediated by courtship song, and cuticular hydrocarbons which function
as pheromones (Etges & Ahrens, 2001). Current
research shows that there are
significant differences in cuticular hydrocarbon components between
mated and
unmated male and female flies when mate choice is allowed.
This indicates that cuticular hydrocarbons are secondary
sexual traits involved in within population mate choice.
Whether
there is a genetic correlation between genes influencing hydrocarbon
expression, life history traits, and mate choice is unknown.
Mate choice in D.
mojavensis is
thought to be governed primarily by female choice, as there has been
no observed male-male competition, or significant differences in male
mating propensity (Brazner & Etges, 1993; Etges, 1998).
Further research is needed to conclusively determine
the specific mechanism of sexual selection in this species,
and
how it is influenced by the environment.
References
ETGES, W.J. 1990. Direction of life
history evolution in Drosophila mojavensis. Pp. 37-56 in J.S.F. Barker,
W.T. Starmer, and R.J. MacIntyre, eds. Ecological and evolutionary genetics of Drosophila.
Plenum, New York.
ETGES, W.J.1993. Genetics of host-cactus response and
life-history evolution among ancestral and derived populations of cactophilic Drosophila
mojavensis. Evolution 47(3):
750-767.
ETGES, W. J. AND AHRENS, M.A. 2001.
Premating isolation is determined by larval-rearing substrates in cactophilic Drosophila
mojavensis. V. Deep geographic variation in epicuticular hydrocarbons among
isolated populations. The American
Naturalist 158:585-597.
HEED, W.B. 1978. Ecology and genetics of Sonoran Desert Drosophila. Pp. 109-126 in P.F. Brussard, ed. Proceedings in the life sciences, ecological genetics: the interface. Springer, New York.
HEED, W.B. 1982. The origin of Drosophila
in the Sonoran
Desert. In Barker,
J.S.F. & Starmer, W.T. (eds). , Ecological Genetics and Evolution: The
Cactus-Yeast-Drosophila Model System, Academic Press, Sydney, pp. 65-80.
JOHNSON, W.R. 1980. Chromosomal
Polymorphism in the Desert-Adapted Species, Drosophila mojavensis. Ph.D. dissertation, University
of Arizona, Tucson.
KOEPFER, R. 1987a. Selection for
sexual isolation between geographic forms of Drosophila mojavensis. I. Interactions between the selected forms.
Evolution 41:37-48.
KOEPFER, R. 1987b. Selection for
sexual isolation between geographic forms of Drosophila mojavensis. II. Effects of selection of mating
preference and propensity. Evolution 41(6): 1409-1413.
KREBS, R.A. & MARKOW, T.A.
1989. Courtship behavior and control of reproductive isolation in Drosophila
mojavensis. Evolution 43:908-912.
MARKOW, T.A., FOGLEMAN, J.C. & HEED, W.B. 1983. Reproductive isolation in Sonoran Desert Drosophila. Evolution 37:649-652.
MARKOW, T.A., 1991. Sexual isolation among populations of Drosophila
mojavensis. Evolution 45:1525-1529.
ZOUROS, E. AND D’ENTREMONT, C.J.
1980. Sexual isolation among populations of Drosophila mojavensis:
response to pressure from a related species. Evolution 34(3): 421-430.
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