51813_Sturgeon biodioversity an.PDF

range and anthropogenic influences including over-
fishing and habitat alteration. We discuss these fac-
tors and then evaluate the current status of this spe-
cies in the Canadian provinces of Manitoba, Onta-
rio, and Quebec from a genetic perspective. We also
summarize and contrast the overall status in the
United States.

Influences on distribution

Postglacial recolonization

Wisconsinan glaciation was a major feature of the

recent zoogeographic past of North America (Ho-

cutt & Wiley 1986). At its maximum about 18 000

years ago, the Wisconsinan ice sheet covered much

of the extant Canadian range of lake sturgeon.

Those populations that occurred in Canada prior to

the Wisconsinan glacial period either perished or

moved into refuges south of the ice sheet. Enor-

mous amounts of meltwater formed rivers and large

lakes, providing dispersal routes for reinvasion. Pa-

leogeographic evidence suggests that lake sturgeon

reinvaded the northern part of its range via the

Warren, Brule, and Chicago dispersal routes from a

Mississippian Refugium (Mandrak & Crossman

1992).

Distribution of mitochondrial DNA (mtDNA)

genetic types (haplotypes) has been used to infer

postglacial recolonization from refugia and test

conclusions based on paleogeographic evidence

(Billington & Hebert 1991). MtDNA is a closed cir-

cular molecule which is maternally inherited and

not subject to recombination (Moritz et al. 1987).

Nucleotide variation in mtDNA is usually detected

indirectly by cutting the molecule with restriction

endonucleases or by determining nucleotide se-

quences directly. MtDNA retains a history of past

isolation for a longer period relative to nuclear

DNA because of its transmission (Billington &

Hebert 1991). Gene frequencies will diverge in both

the nuclear and mitochondrial genomes once a sin-

gle population becomes subdivided into two iso-

lates because of the evolutionary processes of ge-

netic drift and mutation. This divergence may be

enhanced if the isolates become fixed for different

mtDNA types because of stochastic lineage extinc-

tion (Avise et al. 1987). If the descendants of two

populations originating from different refugia

come into secondary contact after glacial recession,

then gene frequencies at variable loci in the nuclear

genome will homogenize until there is no evidence

of past isolation. However, this process occurs more

slowly in the mitochondrial genome because of its

smaller genetic population size (114) relative to the

nuclear genome, which means that four times as

Table 1. Sampling locations and number of lake sturgeon ana-

lyzed for mtDNA haplotype variation. Fish were collected be-

tween 1989-1993 unless otherwise indicated.

Sampling location n

Hudson Bay-James Bay

Moose River basin 129

Waswanipi River 12 a

12

Lower Nelson R. 10 b

Rainy River 16

Total 179

Lake St. Clair 12

Lake Temiskaming 18

Lake Winnebago 18

Lake Ontario (1873)c 1

Ottawa River (Rockland, Quyon, Gatineau) 17 a

10

Lake of Two Mountains 11 a

St. Lawrence River

Great Lakes-St. Lawrence

LacSaint-Louis 11 a,d

Montreal (1866)c 1

Lac Saint-Pierre 20 a

22

Gentilly 11 a

Quebec City 22

Sturgeon River (Lake Superior) 21

Total 195

Mississippi

Mississippi River (1870)c 1

Flambeau River (Mississippi) 21

Total 22

aData from Guenette et al. (1993), otherwise from Ferguson et

al. (1993) and Ferguson (unpublished).

bSampled a single individual with a different haplotype at a se-

quence position other than 54.

cPreserved specimen housed in the British Museum of Natural

History, London.

dSampled a single individual with a different haplotype as deter-

mined by digestion withBcl1.