Santa Cruz Island Reserve

22 Jun, 07 AM - 23 Jun, 02 AM

An unresolved problem is how sexual dimorphism (SD) (trait differences between sexes) can arise even though the sexes share the same genome [1,2]. Theory predicts that sex chromosomes facilitate this evolution by providing a mechanism for the sex-specific inheritance and regulation of adaptive traits [3]. This ?sex-linkage? hypothesis resolves the problem of how SD can arise when sexes share genomes. However, the importance of sex-linked genes and their relationship, if any, to SD is still unresolved [2,3]. I will test the hypothesis that sexual dimorphism is a sex-linked trait in the waterstrider Aquarius remigis (Insecta, Hemiptera). Existing knowledge of the quantitative genetics and adaptive significance of SD in A. remigis make it a model organism for determining the relationship between sex-linkage and SD [4]. Additionally, their small size and short generation times make them ideal to study in the laboratory. Despite previous research indicating significant morphological and genetic variability in both body size and SD [3,4,5], previous experimental designs have not been sufficient to distinguish sex-linkage from other effects. Research Plan I will use a reciprocal line cross analysis (when two populations with distinct forms of a character are interbred and the presence and form of that character is analyzed in the offspring) of genetically different populations to determine whether body size and SD are due to sex-linked effects [6]. The experiment will be designed based on crosses among populations that differ significantly in mean size and degree of SD. I have chosen 3 streams in southern California: specifically, from the Dawson Los Monos Reserve (DLM), the Sedgwick Reserve (SW), and Santa Cruz Island (SCI). These sites were chosen because SCI waterstriders are and island population that is among the largest and display the least SD in North America, whereas DLM and SW waterstriders represent two mainland populations among the smallest and display the most SD in North America [7]. Experimental design To collect waterstriders, I will travel to the 3 streams in the summer of 2009 and use a hand net to collect individuals to bring them back to the laboratory stream tanks at the University of California Riverside. One generation will be reared in the lab from all initial collected individuals to control for environmental effects [7]. Individuals of the first lab reared generation will be used as the parent (P) generation in the reciprocal cross experiments. The contrast in body size and SD of the 3 populations allows me to create two sets of reciprocal crosses. For the first set, I will collect adult A. remigis from DLM (small size and much SD) and SCI (large size and little SD) to produce the following laboratory crosses: SCImale x DLMfemale and DLMmale x SCIfemale. The second set will be constructed using individuals from SCI and SW (a second small size and a lot of SD) to produce the crosses: SCImale x SWfemale and SWmale x SCIfemale. Fifty individuals of each sex and location will be isolated in stream tanks and allowed to mate according to the above experimental design in two replicate experiments. Eggs will be collected and reared to adulthood, forming the offspring (F1) generation. Adults of both the P and F1 generations will be preserved and measured [7]. I will measure a suite of 9 morphological traits in A. remigis, all of which are heritable and genetically correlated between males and females [3,5]. They also show a wide range of SD, from 70% larger in females to 3 times larger in males, thus making them ideal variables to determine if sex-linkage occurs and the potential relationship between it and SD. Conclusions F1 males from reciprocal crosses do not differ with respect to their autosomes. However, they differ in the origin of the sex chromosomes. We can focus on F1 males and mothers, because A. remigis lack Y chromosomes and have an XX/XO system of sex determination [8]. Therefore, all sex-linked effects must come through the X chromosome. F1 males will express the genes on their X chromosome derived from the population of their mother [6]. Therefore, size of the suite of traits can be compared to the average of the mothers? and fathers? traits to quantify the net effect of sex chromosomes on SD traits. A t-test can be used to determine how the F1 male traits compare to the parental average to assess the impact of sex-linkage on traits with differing degrees of SD. If sex-linkage occurs, we expect the F1 male traits to differ significantly from the average of the mothers? and fathers? traits. However, if the traits are autosomally additive, we expect the F1 male traits to not differ significantly from the average of the mothers? and fathers? traits. This simple design will help elucidate the relationship between sex-linkage and SD from which further studies can test more sophisticated hypotheses using quantitative genetic techniques. Works cited [1] R. Lande 1980 Evolution 34 [2] D Fairbairn and D Roff 2006 Heredity 97 [3] D Fairbairn, W Blanckenhorn & T Szekely 2007(eds) Sex, Size and Gender Roles. Evolutionary Studies of Sexual Size Dimorphism. Oxford University Press, UK [4] R Preziosi & D Fairbairn 1992 Evolution 46 [5] R Preziosi & D Roff 1998 Heredity 81 [6] L Wolfenbarger & G Wilkinson 2001 Evolution 55 [7] D Fairbairn 2005 Am. Nat. 166 [8] J Spence & D Maddison 1986 Proc. Entomol. Soc. Wash. 88

Visit #18258 @Santa Cruz Island Reserve

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Under Project # 9717 | Research

Assessing Sex-Linkage and Sexual Dimorphism in the water strider Aquarius remigis

graduate_student - University of California, Riverside

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Group of 2 Graduate Student Jun 22 - 23, 2009 (2 days)

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Dorm 2 Jun 22 - 23, 2009