Molecular Systematics and Population Genetics of Marine Vertebrates from Brazil

Abstract

This thesis examines the implications of patterns of DNA sequence variation in a variety of marine vertebrate species of ecological and fisheries interest from Brazilian waters. The degree of genetic variation in the marine and riverine dolphin Sotalia fluviatilis from Brazilian waters was investigated. A unique genotype found only in Sutalia from the Amazon River suggests that the freshwater form may be genetically distinct from the marine form. The species is genetically diverse in the marine environment but the occurrence of a common genotype in al1 six coastal locations along the marine coast examined suggests that there is sufficient gene flow in the marine region to prevent local differentiation. A previously unexamined mitochondrial locus. COI (cytochrorne oxidase 1). was used in combination with three other loci to re-investigate phylogenetic relationschips of cetaceans. In this data set the largest sequence yet applied to this problem. The controversial Milinkovitch Hypothesis that sperm whales are more closely related to baleen whales than to toothed whales was not supported. Instead four different clades with different taxonomic rankings (Physeteridae. Ziphiidae. Delphinida. and Mysticeti) were identitied. in agreement with the traditional separation of toothed and baleen whales as distinct clades. Results of the analysis are sensitive to locus combinations and method of phylogenetic reconstruction. The species of angel sharks (Squatina, Squatinidae) endemic to the continental shelf of Southern Brazil constitute a monophyletic group. The recently described species S. occulta was found to be more closely related to S. guggenheim than to S. argentina. This phylogeny helps to explain the evolution of reproductive structures (number of ovarics) and patterns of vertical distribution in the water column (from deep to shallow waters) of squatinid sharks. Red snapper (Lutjanus purpureus) shows high genetic diversity off the coast of northen Brazil. Two genotypic clades have been identified. one of which occurs northwest and the other southeast of the discharge of the Amazon River mouth. This is in agreement with recent morphological and reproductive studies which suggest that L. purpureus on the continental shelf of northern Brazil comprises two stock units occupying relatively segregated territories. detined by differences in salinity and temperature. In contrast. the low genetic diversity of yrllowfin tuna (Thunnus albacares) from northeastern Brazilian waters agrees with the hypothesis that only a single stock of yellowfin tuna occurs in the southwest equatorial region of the Atlantic Ocean. The genetic hormogeneity of T. albacares in this area suggests that there is sufficient gene flow in that area to prevent development of local stocks.