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 BMSC ALUMNI

HOME > Research > Club Copepod

The Anholt Lab at BMSC:
Evolution of sex ratio variation in the copepod Tigriopus californicus

Dr. Brad Anholt, Dr. Heather Alexander, Dr. Jean Richardson, Investigators

This variation in offspring sex ratio is larger than the binomial expectation and there is evidence that females produce male-biased clutches at higher temperatures. Voordouw, Robinson & Anholt (2005) also show that sex ratio is heritable and is transmitted primarily through the paternal line (Voordouw & Anholt 2002; Voordouw et al. 2005). We are currently trying to establish the mechanism by which this occurs.

Tigriopus life stages (photo by T. MacKeracher)

The harpacticoid copepod, Tigriopus californicus, lives in splash zone tide pools; an extreme habitat where temperature and salinity varies widely. Field work on Tigriopus has shown that populations fluctuate drastically from summer to winter and that the sex ratio (proportion of males and females) in natural populations often deviates from 50:50 (Figure 1).

The project has three parts. First, characterize naturally occurring variation in the primary sex ratio of Tigriopus californicus, among families, both within and between geographically isolated sites, and over time in Barkley Sound, British Columbia.

Figure 1. The family sex ratio (proportion of males in a single clutch) for a single tide pool over a 10 month period. Females were collected from the field and their families riased in the lab. Each circle represents a family, and family size is relative to circle size.

The second part is to examine the sex-determination mechanism of Tigriopus californicus.  If sex determination is polygenic we should be able to select for both extreme male-bias and extreme female-biased populations.  A return to sex ratio variation larger than the binomial expectation (as observed in natural populations) in subsequent crosses can be interpreted as evidence supporting the theory of polygenic sex determination in this species. Currently we have female and male biased lines established in the laboratory.

Figure 2. Tigriopus populations were selected high and low proportion of males for 8 generations: >0.7 (male bias line in blue) and <0.3 (female biase line in red), control line in black.

Third, during the summer and fall of 2011, male and female biased lines (above) were crossed with male and female biased lines from a genetically divergent T. californicus population (San Deigo). F2's were generated, and we are currently in the process of genotyping the F2's for QTL analysis.

Stay tuned!

People

, Principal Investigator, BMSC Director.

, Post Doc. See my web site.

, Post Doc. Jean's research focus has been on freshwater communities, especially amphibians. She also has a passion for statistics and this combined with her interest in ecological modelling has enabled her to generate computer-intensive statistical methods for difficult to analyze data.

Travis Tai, graduate student, is testing Fisher's sex ratio principle (the frequency dependent evolution of sex ratio) in Tigriopus californicus, which exhibits heritable variation for the primary sex ratio.

Stephanie Keightley, research assistant, is helping with the QTL project, and is currently examining cannibalism by females on newly hatched nauplii.

Amy McConnell, Christina Buelow and Andrew Chapelsky, research assistants, counted and sexed a b-zillion copepods for the QTL project during the fall 2011.

Tracy MacKeracher, undergraduate honour student, examined colonization and dispersal of copepods in the field during the summer of 2010. Her field study provided tantalizing evidence for social mediation of sex determination.

Marie Vance, research assistant funded by NSERC, helped launch the field surveys and selection experiments in the summer and fall of 2009. She recently completed her MSc in Neuchatel, Switzerland studying plant evolution. Marie returned to our lab for the summer of 2011 to follow up on Tracy's work; to test whether social mediation of sex determination of T. californicus occurs under laboratory conditions.

Shanti Davis, research assistant, has been a big help over the winter optimizing laboratory procedures and couting a zillion copepods. She conducts sea bird research in Alaska during the summer, and will be starting an NSERC-funded Master's degree January 2011 at Memorial University.

Melissa MacDougald, summer undergraduate student, continued a group project (started in the Marine Behavourial Ecology class with students Travis Tai, Pierce Deck), "Mate selection in Tigriopus californicus: size of males over multiple populations as a function of reproductive success".

Background: Male Tigriopus californicus clasp immature females prior to their terminal molt, at which time the female is inseminated, then released. Genetic markers indicate that each female mates only once during her lifetime, thus pre-copulatory mate guarding can be interpreted as a strategy to ensure a potential mate has not been previously inseminated. Since males are capable of multiple mating, under most conditions of population sex ratio, this mating system results in low availability of unclasped, developmentally advanced females. As female ratio decreases, males must clasp successively younger females in order to obtain successful insemination (Burton 1985). This situation might suggest that a second strategy of male-male competition for advanced stage females may take place, where males may try to dislodge currently clasped females.

Figure 3. Melissa examined 20 different natural populations with varying sex ratios and measured male body size (length and width) of paired (in pre-copula) and unpaired males. She found that as the proportion of males increase in a population, the size of males in pre-copula was larger than unpaired males, suggesting that size assortative mating occurs, particularly when females are rarer in the population.

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