Complex
population structure
In
migratory fishes, the resolution of populations (and corresponding management
units) can be confounded by two factors:
1 Migratory overlap, in which
populations mingle in feeding habitats or during migrations. Examples of such
overlap can be found in the anadromous Sockeye Salmon (Oncorhynchus nerka;
Grant et al. 1980) and Striped Bass (Morone saxatilis; Wirgin et al.
1997), as well as marine species such as the Bluefin Tuna (Thunnus thynnus;
Carlsson et al. 2007) and possibly
cod (Gadus
morhua; Svedäng et al. 2007). When independent breeding populations overlap
at shared feeding habitats, a critical question is whether genetic exchange
occurs. If fish are not breeding during the period of overlap, those
populations could be isolated management units.
2 Sex-biased dispersal, in which gene
flow between populations is accomplished primarily by one gender. For many
mammals and birds, males disperse prior to reproduction, while females remain
in natal areas (Greenwood 1980).
Both
population overlap and sex-biased dispersal are common in migratory marine
fishes. Female site fi delity can be countered by opportunistic mating by
males, so that each gender yields a different population genetic signal. This
is known as complex population structure (Bowen et al. 2005), and the
most common outcome is that female-inherited mtDNA shows population structure
while biparentally inherited nDNA surveys show no structure (Goudet et al.
2002). This pattern is apparent in the Brook Charr (Salvelinus fontinalis;
Fraser et al. 2004), Patagonian Toothfish (Dissostichus eleginoides; Shaw et al. 2004), and
Shortfin Mako Shark (Isurus oxyrinchus; Schrey & Heist 2003). In a
survey of White Sharks (Carcharodon carcharias) in the Indian Ocean, the
mtDNA sequences reveal significant population structure (Fst =0.81 between South Africa and Australia), while a
microsatellite survey indicated a single population (Pardini et al. 2001). For
these cases, dispersal by males can readily explain the lower population
structure registered in nDNA relative to mtDNA.
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