The nuclear gene X-src is a member of the tyrosine-kinase class of proto-oncogenes whose
normal product is localized within the cytoplasm of the cell. The X-src gene has been used
in only a few phylogenetic studies, each focusing on systematics of killifishes
(Cyprinodontiformes). The present study is an attempt to examine the phylogenetic utility
of X-src for uncovering relationships of representative cichlid fishes, especially the
cichlids of Middle America. The family Cichlidae is a species-rich group of tropical
freshwater fish made up of more than 1000 species which show a Gondwanan pattern of
distribution. Cichlid fish have been the focus of numerous studies ranging from behavioral
to biogeographical to systematic in nature. Particular emphasis has focused on the cichlids
of the African Great Lakes and the explosive adaptive radiation of this group. However,
Neotropical cichlids have received considerably less attention than their African
counterparts. Our findings regarding the utility of X-src concur with those of previous
phylogenetic analyses showing the exons of X-src to be highly conserved and useful mostly
for revealing deep relationships among taxa. Like previous X-src studies, we also found
the intron sequences of the gene to be variable in length and difficult to align across
distantly related taxa but they provided useful information for resolving relationships
among more closely related taxa. The X-src phylogeny supports the monophyly of Neotropical
cichlids and cichlasomines groups A (=heroines) + B (=cichlasomines). A highly resolved
tree is obtained within the heroines but little support is evident for most nodes based
on the low number of unambiguous substitutions. The X-src gene is likely to be quite
useful for resolving deep phylogenetic relationships such as those among major groups of
actinopterygian fishes.