Abstract
The cosmopolitan freshwater pulmonate snail Physa acuta hybridizes readily with Physa carolinae in the laboratory, although their F1 progeny are sterile. The two species differ qualitatively in shell shape, the former bearing a more globose shell and the latter more fusiform. We performed a hybridization experiment, measuring a set of 14 traditional (linear) and landmark-based shell morphological variables on even-aged parents and their offspring from both hybrids and purebred control lines. Parent-offspring regression yielded a strikingly high heritability estimate for score on the first relative warp axis, h2 = 0.819 ± 0.073, a result that would seem to confirm the value of geometric morphometrics as a tool for retrieving evolutionary relationships from gastropod shell form. Score on the second relative warp axis was also significantly heritable (h2 = 0.312 ± 0.123), although more moderate, as were scores on second principal components extracted from traditional measurements (correlation h2 = 0.308 ± 0.069, covariance h2 = 0.314 ± 0.050). Although score on the first relative warp axis was significantly correlated with centroid size (p < 0.001), scores on none of the three second axes were so correlated. This result suggests that second axis score might prove especially useful for estimating genetic divergence among mixed-age populations of gastropods sampled from the field.
Highlights
The methods by which the biological information conveyed in gastropod shell morphology have been retrieved and analyzed are as diverse as the origins of that information itself
What fraction of the variance in shell morphology qualitatively observable in a comparison of Physa acuta and P. carolinae might be additively heritable? What analytical techniques might maximize the heritable fraction recovered? Are modern geometric approaches so superior to more traditional linear methods that all previous research results are discredited? Here we report the results of acuta x carolinae hybridization experiments, analyzing shell morphological data collected on sets of even-aged parents and offspring with both traditional and landmark-based techniques
The first principal component (PC) extracted from the correlation matrix of these six variables across the 84 measurements accounted for 92.1% of the variance, all variables loading positively and evenly, as expected for such data
Summary
The methods by which the biological information conveyed in gastropod shell morphology have been retrieved and analyzed are as diverse as the origins of that information itself. Theoreticians [1] have explored modeling approaches to compare the vast diversity of gastropod shell form naturally observed to that which might be possible. With the advent of multivariate morphometrics in the 1970s [2,3], evolutionary biologists began analyzing gastropod shell morphology with principal components [4], discriminant functions [5,6], and factor analysis [7]. In the early 1990s, the study of morphometrics was revolutionized by the development of new analytical methods designed to capture the geometrical relationships among sets of digitized landmarks [8]. The first applications of geometric morphometric techniques to gastropod
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