Ontogeny and phylogeny in the northern swordtail clade of Xiphophorus

It has been hypothesized that morphological diversity within clades can be generated by simple alterations of shared developmental programs. However, few studies have examined changes in heterochrony, the rate and timing of d evelopmental events, in an explicitly phylogenetic context. We studied how developmental patterns have changed phylogenetically in the northern swordt ail clade of Xiphophorus. We reared individuals of an outgroup and seven of nine species in the clade and followed their development for similar to 30 0 days. For each individual, we used nonlinear regression to estimate three growth parameters: growth rate, adult body size, and age of cessation of g rowth. We estimated sword growth rates in males by linear regression. We th en used the means of these growth parameters to construct standard growth c urves for each species and to study growth patterns in a phylogenetic conte xt. A combined phylogeny was constructed from both phenotypic and DNA. sequ ence data. The phenotypic data set, compiled from the literature, consisted of 86 morphological, pigmentation, behavioral, and random amplified polymo rphic DNA characters, many of which had not been used before for phylogenet ic analysis. DNA sequence data from three genes for a total of 1284 bases w ere also obtained from the literature and included in the analysis. Relatio nships between growth parameters were examined by phylogenetically independ ent contrasts in relation to seven different phylogenies based on the most- parsimonious trees generated from the phenotypic, DNA sequence, and combine d data sets; this allowed us to identify relationships between variables th at were not sensitive to ambiguities in Xiphophorus phylogeny. Our analysis revealed statistically significant correlations between female body size a nd male body size, and between female growth rate and male sword growth rat e, for all seven phylogenies. Marginally statistically significant relation ships were also identified between female body size and female growth rate, and between female growth rate and male body size. We relate these relatio nships to what is known about the ecology, genetics, and behavior of Xiphop horus to better understand the evolution of growth patterns of both the bod y as a whole and the sword in particular. The relationship of these data to the evolution of swords is discussed.