Biology Reference
In-Depth Information
Highly Similarity between the Baiji and Finless Porpoise: Convergent
Evolution?
It was interestingly noted that certain identical alleles were shared by the baiji and finless
porpoise in China waters at all three MHC loci. Of all the MHC alleles identified in this
chapter and those reported by Yang et al. (2005), Hayashi et al. (2006) and Xu et al. (2007),
six pairs of alleles, i.e., one at
DRA
, two at
DQB
, and three at MHC-I, were identical between
the two species. Each of these identical alleles was identified from at least two individuals or
independent clones. For example,
Live-DRA*01
and
Neph-DRA*01
are two identical alleles,
the former of which was identified from 30 clones of 10 baiji samples, whereas the latter of
which was detected in 94 clones of 35 finless porpoises. At the MHC-I locus, three pairs of
identical alleles in both species were from 54 clones in 13 baiji individuals and 18 clones in
five finless porpoises, respectively. Two pairs of identical
DQB
alleles were shared by 16
baiji individuals and four finless porpoises, respectively. In addition to the identical alleles,
other alleles of the baiji and finless porpoise had highly interspecific similarity as shown in
Tables 3 and 4. Actually, mitochondrial control region sequences were determined from the
same DNA extractions, and all these sequences correctly correspond to either baiji or finless
porpoise, without any haplotype shared by both species (Yang et al., 2003, 2008). For this
reason, the possibility that the identity or similarity between the baiji and finless porpoise
may be due to sample contamination, ―PCR artifacts‖, or chance, should be excluded.
Up to now, cases of total identity amongst MHC alleles from different species have been
reported (Leuchte et al., 2004; Otting et al., 2002; Suárez et al., 2006; Huchard et al., 2006),
but most of them are restricted to congeneric species and rarely from above genus level
(Suárez et al., 2006; Otting et al., 2002; Huchard et al., 2006). The identity or high similarity
between different but closely related species, as a result of long-term effect of selection on
MHC, was usually explained by trans-species mode of evolution (Sena et al., 2003; Huchard
et al., 2006). Trans-species evolution refers to polymorphism that predates speciation events,
whereby allelic lineages are passed from species to species and persist over long periods of
evolutionary time (Klein, 1987). This was evidenced by this chapter that some alleles from
harbor porpoises (
Phph-a
) and vaquita (
Phsi-DQB*01
) clustered with those of finless
porpoises (Figure 2b). Other evidence came from the
DRA
data. Forty-five individuals of
other cetacean species from Pontoporiidae, Phocoenidae, and Delphinidae, were also
examined at the
DRA
locus for comparison, which revealed that the alleles shared between
the baiji and finless porpoise (i.e.,
Live-DRA*01
and
Neph-DRA*01
) were also found in
species of Phocoenidae and Delphinidae (data not shown).
However, it is difficult to explain the identity and high similarity between distantly
related species, e.g., the baiji and finless porpoise, using the trans-species mode of evolution.
The two species are highly divergent with each other, with the baiji included in Lipotidae of
the superfamily Lipotoidea (de Muizon, 1988; Yang et al., 2002) and the finless porpoise in
Phocoenidae of the superfamily Delphinoidea (Rice, 1998), respectively. As suggested by
some other authors (Andersson et al., 1991; Gustafsson & Andersson, 1994; Kriener et al.,
2000), identity and similarity between distantly related species can be explained by
convergent evolution. Unlike trans-species evolution, the identity and similarity that are
shared in the case of convergent evolution are not the result of evolution from a common
ancestor, but typically explained as the result of common adaptive solutions to similarly
environmental pressures. As for the baiji and finless porpoise, it is well known that they are
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