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Dolphin Phylogeny
Introduction
Our research question was to learn how river dolphins are related
to ocean dolphins and other cetaceans. Our hypothesis was that
biogeography would show a higher correlation to common
ancestry than would habitat. A secondary question was to use
two different gene trees for the same species to see if similar
phylogenies would be generated by each.
April Maskiewicz, Dawne Page &
Kerry Fulcher
Hypothesis: Individual species of river dolphins
will be more closely related to geographic
neighbors than to each other.
Fig. 1 16S rRNA unrooted tree
Methods:
In order to test our hypothesis, we chose to compare cetacean species with
particular biogeographic ranges. Due to the limited amount of sequence
available from river dolphins, we could only use sequences from
mitochondrial DNA. We chose nucleic acid sequence from the 16S rRNA
gene and amino acid sequence from cytochrome b for the species listed in
Table 1.. We chose hippo and horse sequences as potential outgroups.
Sequences were collected and edited for common length on the Biology
Workbench 3.2 via internal (Workbench) or external (NCBI) database
searches. Alignments were made and unrooted sequence similarity trees
generated using Clustal W and DrawTree respectively. Finally, an
evolutionary distance matrix was generated for the Cytochrome B data
Common Dolphin
using PROTDIST (Fig. 3)
Amazon RD
Indus RD
Fig. 3 Evolutionary Distance Matrix form the multiple sequence comparison of
Cytochrom B protein sequences. Data were artificially grouped into Low (green),
Medium (Yellow) and High (Red) percent divergence to highlight relationships.
Results:
Two very different trees were generated. The rRNA gene tree (Fig. 1) showed 3
clusters: horse, Amazon river dolphin with all of the whales, and Asian river dolphins
with the ocean dolphins and hippo. These data imply separate origins for the Amazon
vs. Asian river dolphins. However, the cytochrome b tree (Fig. 2) did not imply such a
simple conclusion. The Beluga and Humpback whales were closely related to the
ocean dolphins (93-94%). The Humpback whale was also more closely related to the
Indian river dolphins (92%) than to the Amazon & Yangtze river dolphins (88%). In
contrast, the Humpback whale was more closely related to the Amazon river dolphin in
the 16S rRNA phylogeny.
Conclusion:
Our tentative conclusion is that the river dolphins in India, the Amazon, and the Yangtze
have separate origins, but these origins are unclear from the data, which illustrate the
inherent difficulties of constructing phylogenies from several gene trees.
Fig. 2 Cyt B unrooted tree
Table 1. Species comparison
using Cyt B protein sequences
and 16S rRNA sequences