Increased variability and reduced phenotypic robustness in clonal Drosophila mercatorum
Quantitative genetics predicts that reducing genetic variability should reduce phenotypic variability, a common principle that underlies the widespread use of inbred, isogenic, and clonal animals in biomedical research. Yet extreme homozygosity can also expose recessive deleterious alleles and detoriate developmental buffering, potentially destabilizing rather than canalizing phenotypic outcomes. We tested this conflict directly in Drosophila mercatorum, a facultatively parthenogenic fly species in which pronuclear duplication results in fully homozygous, clonal offspring already after a single generation. Contrary to the expectation that genetic uniformity should reduce phenotypic variability, clonal parthenogenic flies showed changes in trait means, trait-dependent changes in interindividual variation, increased fluctuating asymmetry, and, most consistently, reduced behavioral and developmental canalization across a broad set of parameters. These effects on phenotypes spanned visually guided locomotion, circadian activity, wing morphology, bristle patterning, eye anatomy, brain volume, and serotonergic neuron numbers and were associated with reduced survival under environmental stress. Inbreeding of sexual D. mercatorum reproduced several of these phenotypic changes, whereas a single generation of outcrossing restored phenotypic robustness and the resulting F1 hybrids even exceeded the wild-type controls in several traits, consistent with a hybrid vigor effect. Together, these experiments identify loss of heterozygosity, rather than clonality per se, as the primary driver of developmental instability. Our findings show that genetic uniformity achieved through homozygosity can amplify stochastic phenotypic divergence by weakening developmental canalization, and suggest that the genetic context in which experimental standardization is achieved, especially heterozygosity, matters as much as the degree of genetic uniformity itself.