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HOME > About > News and Features > Club Copepod

Research Feature:
Evolution of sex ratio variation in the copepod Tigriopus californicus

Dr. Brad Anholt, Dr. Heather Alexander, Investigators

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).

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.

The project has two 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: >0.7 (male bias line) and <0.3 (female biase line).

People

Dr. Brad Anholt, Principal Investigator, BMSC Director.

Dr. Heather Alexander, Post Doc. See my web site.

Tracy MacKeracher, undergraduate honour student, is currently working on colonization and dispersal of copepods in the field.

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

Marie Vance, research assistant funded by NSERC, helped launch the project in the summer and fall of 2009. She is currently working on her PhD in Neuchatel, Switzerland studying plant evolution. Best of luck to Marie!

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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