Journal of Heredity Advance Access originally published online on December 23, 2004
Journal of Heredity 2005 96(2):114-123; doi:10.1093/jhered/esi014
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Genetic Consequences of Many Generations of Hybridization Between Divergent Copepod Populations
From the Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089-0371. H. V. Feaman is currently at the School of Criminal Justice and Criminalistics, California State University, Los Angeles, CA 90032. J. S. Harrison is currently at the Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0202. C. C. Timmerman is currently at Methinks.com, 971 W. 30th St., Los Angeles, CA 90007
Address correspondence to Suzanne Edmands at the address above, or e-mail: sedmands{at}usc.edu.
Crosses between populations of the copepod Tigriopus californicus typically result in outbreeding depression. In this study, replicate hybrid populations were initiated with first generation backcross hybrids between two genetically distinct populations from California: Royal Palms (RP) and San Diego (SD). Reciprocal F1 were backcrossed to SD, resulting in expected starting frequencies of 25% RP/75% SD nuclear genes on either a pure RP cytoplasmic or a pure SD cytoplasmic background. After 1 year of hybridization (up to 15 generations), seven microsatellite loci were scored in two replicates on each cytoplasmic background. Frequencies of the rarer RP alleles increased significantly in all four replicates, regardless of cytoplasmic source, producing a mean hybridity of 0.97 (maximum = 1), instead of the expected 0.50. Explicit tests for heterozygote excess across loci and replicates showed significant deviations. Only the two physically linked markers showed linkage disequilibrium in all replicates. Subsequent fitness assays in parental populations and early generation hybrids revealed lower fitness in RP than SD, and significant F2 breakdown. Computer simulations showed that selection must be invoked to explain the shift in allele frequencies. Together, these results suggest that hybrid inferiority in early generations gave way to hybrid superiority in later generations.
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