Assessing niche differences of sex, armour and asymmetry phenotypes using stable isotope analyses in Haida Gwaii sticklebacks

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Identifying phenotype-specific selective landscapes within populations continues to challenge evolutionary biologists in studies of adaptive variation. We explore here the potential application of carbon and nitrogen stable isotope ratios of tissues as time-integrated proxies of niche space among sex, armour and asymmetry phenotypes within an endemic population of giant threespine stickleback from Haida Gwaii, western Canada. Muscle tissues were extracted from 289 stickleback collected from Drizzle Lake, taken in transects during June 1981, June 1982, September 1982 and June 1983 and isotopic ratios of 15 N/ 14 N ( δ 15 N) and 13 C/ 12 C ( δ 13 C) determined by continuous-flow isotope ratio mass spectrometry. Among all fish, δ 15 N values, which reflect relative trophic level, ranged from 6.5‰ to 10.6‰ while δ 13 C, which reflects reliance on different carbon pathways of primary producers, ranged from −30.5‰ to −27.5‰. The sexes did not differ in δ 15 N but females were significantly enriched in δ 13 C relative to males among all samples. In each transect, lateral plate phenotypes differed in δ 15 N, with higher plate counts generally enriched in 15 N. δ 13 C did not vary among plate pheno-types. Approximately 50% of the adult population exhibit lateral plate asymmetries usually with one plate different between sides. Sticklebacks that were asymmetric (absolute asymmetry) did not differ from symmetric fish for either δ 15 N or δ 13 C signatures. However, this result masked a significant and consistent effect of signed asymmetries (right side – left side), with δ 15 N enrichment among right-biased compared to left-biased asymmetric. These unexpected results indicate the presence of subtle and previously unrecognized niche differences in lateral plate number and asymmetry phenotypes and are consistent with the predictions of niche-width and adaptive variation hypotheses. The results also suggest the wider application of isotopic techniques to elucidate phenotype-specific ecological and selective landscapes.