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Abstract
In addition to natural selection, adaptive evolution requires genetic variation to
proceed. Yet the G-matrix may have limited 'genetic degrees of freedom', with certain
combinations of trait values unavailable to evolution. Such limitations are often
referred to as genetic constraints. Unfortunately, clear predictions about when to
expect constraints are rarely available. Therefore, we developed an experimental system
that provides specific predictions regarding constraints. Such tests are important
as disagreements persist regarding the evidence for genetic constraints, possibly
due to differences in methodology, study system or both. Numerous measures of genetic
constraints have been suggested, and generally focus on whether some axes of G have
eigenvalues=~0, indicating a lack of genetic variance.The mutation Ultrabithorax1
causes a mild homeotic transformation of segmental identity. We predicted that this
mutation would induce a genetic constraint due to this homeosis. We measured genetic
co-variation for a set of traits in a panel of strains with and without Ubx1. As expected,
Ubx1 induced homeotic transformations, and altered patterns of allometry. Yet, no
changes in correlational structure nor in the distribution of eigenvalues of G were
observed. We discuss the role of using genetic manipulations to refine hypotheses
of constraints in natural systems.