Evolution of Development: Needlefish Jaws
Evolutionary change in developmental timing (heterochony) is an important source of morphological modification and innovation. I am using a phylogenetic approach to understand heterochrony and jaw ontogeny in the teleost order Beloniformes, which traditionally includes the needlefishes (family Belonidae), halfbeaks (family Hemiramphidae), sauries (Scomberesocidae), flyingfishes (Exocoetidae), and ricefishes (Adrianichthyidae). This diverse clade offers a superb system for examining the evolution of ontogeny because many of its taxa show dramatic growth-related changes in jaw structure.
Potamorrhaphis guianensis, a freshwater South American needlefish
Larval needlefishes have short jaws of equal length. As juveniles, the lower jaw elongates first, producing a distinctive “halfbeak” stage with jaws of unequal length. Finally, in most species, the upper jaw extends to produce the “forceps-jawed” adult form. A closely related family, the halfbeaks, have an adult jaw morphology that is quite similar to that of juvenile needlefishes. In fact, juvenile needlefishes have occasionally been described erroneously as new species of halfbeaks. Alternative hypotheses have been proposed to explain these ontogenetic and phylogenetic patterns. Severtsov (see Gould, Ontogeny and Phylogeny, 1977) suggested that beloniform phylogeny was recapitulated in the ontogeny of needlefishes. In this scenario, needlefishes are the most derived beloniforms, and their “halfbeak” ontogenetic stage reflects a halfbeak ancestry. The alternative hypothesis, championed by de Beer (Embryos and Ancestors, 1940) is that halfbeaks are actually paedomorphic needlefishes. In this case, the “halfbeak” adult morphological condition would be derived relative to the “forceps-jaws” of adult needlefishes.
I am testing this hypothesis using datasets that include morphological and molecular (both nuclear and mitochondrial) characters. My ongoing analyses conclusively show that the “forceps-jawed” morphology of adult needlefishes is derived relative to the “halfbeak” morphology of adult halfbeaks (Lovejoy, 2000; Lovejoy et al., 2004). Thus, halfbeaks are not paedomorphic needlefish derivatives. My results are also providing valuable taxonomic insights, including the finding that sauries are nested within the needlefishes, and the Southeast Asian freshwater halfbeaks (Zenarchopteridae) are a distinct clade that is most closely related to needlefishes/sauries; this work is proceeding in collaboration with Bruce Collette (Systematics Laboratory, National Marine Fisheries Service).
As I continue to add needlefish species and close relatives to my analysis, I have a long term goal of investigating the molecular and morphogenetic modifications that produce these developmental changes, as well as the ecological circumstances that make them likely. My analysis of heterochrony will thus span macroevolutionary, microevolutionary, and organismal level perspectives.[click on the images below]