Big Picture - Our primary interest is understanding the evolution and genetic basis of adaptive phenotypes (i.e., linking genotype with phenotype; see Huestis and Marshall 2009, Bioscience). Over the last decade and a half we have investigated a range of adaptive phenotypes including body size, longevity, fecundity, egg diapause, thermal tolerance, courtship behavior, sperm competition, and oviposition. We have been especially interested in two main topics: (1) the genetics of speciation and (2) the dynamics of genes and phenotypes that exhibit environmental clines. Although we have used a variety of animals in our research, our focal system is the Allonemobius socius complex of crickets.
Allonemobius crickets and the i5k genome project
i5k wikis: currently, there are ~20 scientist that support the sequencing of the
Allonemobius socius and Allonemobius fasciatus genomes. Want
to join our group? If so, e-mail me at cricket@ksu.edu.
Why sequence the Allonemobius genome?
(1) 127 publications since 1980 (a well-studied non-model insect)
(2) >3,700 citations from these publications (data from Google Scholar)
(3) Used to study ... i. Evolutionary Biology (77 publications)
ii. Ecology (50 publications)
iii. Evolutionary Genetics (42 publications)
iv. Physiology (40 publications)
v. Behavior (38 publications)
(4) A model for rapid evolution of new species and postmating, prezygotic isolation
(5) Several genomic tools already in place including ...
i. RNAi has been done successfully
ii. EST libraries (based on 454 sequencing) for ...
a. Female reproductive tract (168,690 sequences)
b. Male testis (144,416 sequences)
c. Male ACG (37,521 sequences)
iii. Several linkage maps have been generated
(6) Significant NSF funding since the early 1980s
The Allonemobius socius complex of crickets
The A. socius complex of crickets is comprised
of three species (A. socius, A. fasciatus, and A. sp.
nov. Tex; Marshall 2004, Evolution) . These species
diverged from a common ancestor about 30,000 years ago
(Fig.1) and form two hybrid zones (Fig. 2). - one
between A. socius and A. fasciatus that runs east-west
from New Jersey to Illinois and a second between
-
A.socius and A. sp. nov. Tex that runs north-south
near the Texas-Louisiana boarder (Fig. 2). Research
has shown the hybrid zone between A. socius and
A.fasciatus is mosaic in structure while the zone
between A. socius and A. sp. nov. Tex appears to be
clinal. After decades of research by Dan Howard’s
laboratory and that of his former students and postdocs
(including myself), it is clear that the primary mechanism
of reproductive isolation between these species is
postmating, prezygotic isolation. The specific phenotypes
include the ability of males to induce females to lay eggs
and conspecific sperm precedence. Given these
phenotypes are the main mechanism of reproductive isolation and that divergence is recent, this cricket group makes an excellent system to study the genetics of speciation and not just species differences.
Genetics of speciation and postmating, prezygotic phenotypes
In addition to the above features, the study of postmating,
prezygotic phenotypes in this group of crickets is aided by males
producing a spermatophore - a protein structure that contains all the
sperm and seminal fluid proteins of the ejaculate (FIg. 3 A & B). This
spermatophore is attached to the female during copulation and
serves to transfer the ejaculate to the female. Therefore, the entire
world of male reproductive isolation proteins is packaged by the male
into this visible, easily collected package.
Given that males package “speciation proteins” into the
spermatophore, we have been using comparative proteomic studies
to identify the proteins that are divergent between species as part of
a NSF project (DEB 0746316). Specifically, we are using 2D-DIGE
(Fig. 4 - example 2D-DIGE gel) and mass spectroscopy (MS and
MS/MS) to identify those proteins that are consistently species-
specific across the two hybrid zones (see Marshall et al. 2011 in
Molecular Biology and Evolution for a detailed description of this
process). Once identified, we sequence the underlying gene,
determine if it exhibits species-specific nucleotide variation, assess whether or not species-specific alleles flow across the contact zone, and test for patterns of positive selection acting on the gene. For those genes whose alleles remain species-specific in the face of gene flow in contact zones, we conduct functional analyses using RNAi to identify the phenotype (Fig. 5 - see Marshall et al. 2009, PLoS One).
In the news: see this BioTechniques news article on our lab’s use of RNAi
This work is currently underway and we have begun to explore male ejaculate - female reproductive tract interactions using the same procedures. Using these comparative proteomic techniques and a wide range of biochemical tools, we are on the cusp of identifying the male-female protein interactions that are responsible for reproductive isolation in this group of crickets and thus, those proteins that underlie speciation.
The dynamics of genes and phenotypes that exhibit environmental clines
Genes in the environment:
Some of our more recent work has examined the latitudinal gradients of allele frequencies that exist at many loci. Specifically, we have been studying the IDH-1 locus in A. socius which shows evidence of clines in the distribution of two alleles - a fast allele in southern environments and a slow allele in northern environments (Huestis and Marshall 2006, Ecological Entomology). In addition to these clines, there appears to be fitness advantages to possessing the right genotype in the right environment. Given these data, my former student Dr. Diana Huestis explored the enzyme kinetics of these alleles across a naturally relevant thermal gradient and found that slow alleles performed better in cooler environments and faster alleles did better in hotter environments (Huestis, Oppert, Marshall 2009, BMC Evolutionary Biology). This is one of a handful of studies to link physiological abilities of alleles in various environments with fitness effects and the distribution of alleles across a thermal gradient. This work was highlighted by Ary Hoffman in Faculty of 1000.
Phenotypes in the environment:
We have studied the link between several phenotypes and environmental gradients, with the two most studied phenotypes being egg diapause in cricket and body size in salamanders. With regard to egg diapause, which is a pause in development that enables eggs to survive winter, we have found significant phenotypic plasticity with the greatest degree of plasticity occurring at higher elevations. Moreover, the diapause history of the female (i.e., whether or not she experienced diapause as an egg) can influence the percentage of her eggs that go through diapause (Huestis and Marshall 2006, Oecologia). Lastly, we have begun to study the genetic basis of diapause and phenotypic plasticity within Allonemobius. Using a population genomic approach, we have identified genetic markers that underlie both percentage of diapause eggs laid by females and the degree of phenotypic plasticity in diapause. In total, the genetic markers we identified account for >90% of the variance in both traits.
