Equivalent Survival and Different Development Rates in Reciprocal Apache Trout × Rainbow Trout Hybrids

Hybrid individuals from naturally spawning populations of Apache Trout (Oncorhynchus gilae apache) and Rainbow Trout (Oncorhynchus mykiss) have been reported to hybridize successfully in only one direction. This is suggested by the absence of Rainbow Trout mitochondrial haplotypes in hybrid individuals. By artificially producing reciprocal crosses of Apache Trout and Rainbow Trout, the apparent unidirectional fertilization in naturally spawning hybrid populations was shown not to be the result of gametic incompatibility or early zygote death. Although survival rate differences between the reciprocal hybrid crosses were not significantly different, development rate differed significantly between the groups (P < 0.001). Using time to hatch as the indicator of development rate, Apache female by Rainbow male hybrids had a mean time to hatch seven days faster than Rainbow female by Apache male hybrids with both developing intermediate to the pure crosses. Although this could account for the perceived unidirectional hybridization in naturally spawning populations, it is more likely that some type of temporal or behavioral isolation accounts for the apparent directionality of the hybridization.