Genetic Analysis of Hybrid Zones: Numbers of Markers and Power of Resolution

Abstract

The use of molecular genetic techniques is becoming increasingly widespread in analyses of hybrid zones. Yet, exactly how many molecular markers are required to provide a given degree of resolution remains an open question. We present statistical models that relate the number of markers examined to their power to discriminate between pure species, F1's, and backcross individuals. Our models indicate that only about four or five markers are required to provide a coarse classification of individuals in hybrid zones, whereas upwards of 70 markers are required to discriminate between pure species and advanced backcrosses. The models provide hybrid zone researchers a basis upon which to balance the time and expense of examining large numbers of markers against the magnitude of classification errors when smaller numbers of markers are examined.

Family effects on the growth and survival of congeneric blue mussel larvae (Mytilus edulis and M. trossulus)

“…Because the limited number of markers we used would have led us to misclassify backcross hybrids (especially late-generations) as pure species (Boecklen and Howard 1997), these nominally pure crosses may nevertheless have included individuals that were introgressed. With three codominant markers at loci carrying species-diagnostic alleles (as used in this study), the probability of error (P e ) for misclassifying backcross hybrids as pure species increases with the generation of backcrossing; backcross generation 1, P e = 0.15; 2, P e = 0.45; 3, P e = 0.68; 4, P e = 0.85; 5, P e = 0.90 (Boecklen and Howard 1997). However, with our three markers, we can be confident that an individual we type as "M. edulis" is not an M. trossulus (P e = 0), an F 1 hybrid (P e = 0), or an F 2 hybrid (P e < 0.05).…”

“…However, with our three markers, we can be confident that an individual we type as "M. edulis" is not an M. trossulus (P e = 0), an F 1 hybrid (P e = 0), or an F 2 hybrid (P e < 0.05). See Boecklen and Howard (1997) for a full treatment of these assignment issues.…”

Vegetative and Adaptive Traits Predict Different Outcomes for Restoration Using Hybrids

“…As demonstrated here, discriminant analysis can be used on known hybrids to help evaluate the trajectory of the current hybrid population. Introgression (Zalapa et al, 2009) and hybrid swarm effects can be revealed with detailed genetic analysis, especially with the advance of sequencing technology to produce the required amount of markers (Boecklen and Howard, 1997; Elshire et al, 2011). Yet, genetic analysis often cannot reveal phenotype, which is ultimately what selection acts upon.…”

“…Assessing the hybrid trajectory is therefore critical to the study and management of hybridizing populations, yet is difficult to accomplish using genetic analysis alone. The percent admixture (Boecklen and Howard, 1997), class (Anderson and Thompson, 2002), or entire sequence of a hybrid individual does little to reveal heterosis (hybrid vigor) and other non-additive genetic effects (Chen, 2010). Genetic data also reveal little regarding how differences in physiology and metabolism would affect secondary metabolite production (Cheng et al, 2011) or transgressive phenotypes (Rieseberg and Carney, 1998).…”

Near absence of hybridization between sauger and introduced walleye despite massive releases

“…Sampling error is likely not the cause of the observed pattern; the median sample size was 30 and our sampling scheme was geographically comprehensive. Additionally, there is \1% chance of not detecting hybridization using 11 loci with a sample size of 30 from a hybrid swarm containing 1% admixture from walleyes (Boecklen and Howard 1997). Finally, with the exception of the sample from Boysen Reservoir (sample 19), sampling was random with respect to both species.…”