Research Document - 2011/008

Genetic and Phenotypic Diversity and Divergence in Sympatric Lake Utopia Rainbow Smelt (Osmerus mordax)

By I.R. Bradbury, R. Bradford, and P. Bentzen

Abstract

The Rainbow Smelt complex in Lake Utopia New Brunswick represents an independent occurrence of multiple sympatric ecological forms, the identification and status of which remains unresolved. Here we re-evaluate the presence of phenotypic and genetic divergence within Lake Utopia smelt using samples of spawning individuals collected in 1980-1981, 1990-1991, 2002-2003, and 2010. Phenotypic divergence was examined using several meristic traits (gill raker, and fin ray counts) and linear morphometric measurements. Gill raker counts generally support the presence of two groups of smelt, whereas variation in body length reveals three distinct modes. Principle Component Analysis (PCA) and Quadratic Discriminant Function (QDFA) analysis of linear morphometrics support hypotheses of two or three size-based groups (98-99% accurate assignment) and the statistical removal of size significantly reduced (~65%) assignment success. Data on 10 microsatellite loci spanning 20 years consistently support a hypothesis of two discrete populations. Principle Coordinate Analysis and Bayesian clustering using STRUCTURE revealed the presence of two populations, one associated with the two smaller body length modes, and the other consisting of individuals greater than ~170 mm fork length. Evidence of hybridization between the forms was observed in both morphs. Moreover, multiple small-form individuals were identified with ‘large form’ genotypes. Overall, the results support the presence of two genetically distinct forms in Lake Utopia, with evidence of phenotypic divergence within the small form. The recurring presence of hybrid individuals and gene flow from the large to small forms suggests that the stability of the pair will likely depend on the persistence of suitable local environmental conditions, and presents the possibility of introgressive meltdown of the forms if conditions change. Further study of the rate of hybridization, the fate of hybrids, and the selective landscape associated within the lake are needed to evaluate the status and allow continued conservation of smelt in Lake Utopia.

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