Postzygotic isolation involves strong mitochondrial and sex-specific effects in
Tigriopus californicus,a species lacking heteromorphic sex chromosomes |
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Authors: | B R Foley C G Rose D E Rundle W Leong S Edmands |
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Institution: | 1.Department of Biological Sciences, University
of Southern California, Los Angeles, CA, USA |
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Abstract: | Detailed studies of the genetics of speciation have focused on a few model systems,
particularly Drosophila. The copepod Tigriopus californicus offers an
alternative that differs from standard animal models in that it lacks heteromorphic
chromosomes (instead, sex determination is polygenic) and has reduced opportunities for
sexual conflict, because females mate only once. Quantitative trait loci (QTL) mapping was
conducted on reciprocal F2 hybrids between two strongly differentiated
populations, using a saturated linkage map spanning all 12 autosomes and the
mitochondrion. By comparing sexes, a possible sex ratio distorter was found but no sex
chromosomes. Although studies of standard models often find an excess of hybrid male
sterility factors, we found no QTL for sterility and multiple QTL for hybrid viability
(indicated by non-Mendelian adult ratios) and other characters. Viability problems were
found to be stronger in males, but the usual explanations for weaker hybrid males (sex
chromosomes, sensitivity of spermatogenesis, sexual selection) cannot fully account for
these male viability problems. Instead, higher metabolic rates may amplify deleterious
effects in males. Although many studies of standard speciation models find the strongest
genetic incompatibilities to be nuclear–nuclear (specifically X
chromosome–autosome), we found the strongest deleterious interaction in this system
was mito–nuclear. Consistent with the snowball theory of incompatibility
accumulation, we found that trigenic interactions in this highly divergent cross were
substantially more frequent (>6 × ) than digenic interactions. This alternative
system thus allows important comparisons to studies of the genetics of reproductive
isolation in more standard model systems. |
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Keywords: | QTL reproductive isolation speciation copepod nuclear– mitochondrial interaction |
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