loss occurred through genetic drift or sampling variation only. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Saccostrea glomerata; Heterozygosity; Allozymes; Oysters; Australia
1. Introduction
Ž Production of the native Sydney rock oyster, Saccostrea glomerata note species
Ž ..
name change from S. commercialis, see Anderson and Adlard 1994 , like many other native oysters, has declined in recent years. The Pacific oyster, Crassostrea gigas, now
Ž .
dominates world oyster production Shatkin et al., 1997 . Sydney rock oyster production has declined from about 14.5 million dozen oysters in the 1970s to 8.5 million dozen
Ž .
oysters in 1995r1996 NSW Fisheries, 1998 . Farming of the Sydney rock oyster, Ž
which relies on natural spatfall, began on the Australian east coast New South Wales .
Ž and southern Queensland in the 1870s and on the west coast in the early 1980s Nell,
. 1993 . Declining production rates have resulted from oyster mortalities by infection of
Ž protoctistan parasites
winter mortality, Mikrocytos roughleyi, and QX, Marteilia .
sydneyi; Nell, 1993 , adverse effects of viral outbreaks among consumers, and labour-in- tensive methods that have reduced economic viability.
In an attempt to increase profitability and to meet competition from the faster growing Pacific oyster in Tasmania, South Australia and New Zealand, a selective
Ž .
Ž .
breeding program using mass selection was established in 1990 Nell et al., 1996 . Equal numbers of oysters were taken from each of four estuaries in New South Wales:
Ž
X X
. Ž
X X
. Georges 3280 S, 15180 E and Hawkesbury 33830 S, 151815 E Rivers, Wallis Lake
Ž
X X
. Ž
X X
. 32810 S, 152829 E , Port Stephens 32845 S, 152810 E . Efforts were made to ensure
that only locally caught oysters were sampled, not those that had been transferred from another estuary. Oysters from each of the four estuaries were divided equally between
Ž .
eight groups for spawning 100 oystersrgroup . Of the 100 oysters in each of the eight spawning groups, not all of the 100 oysters spawned–only those that spawned profusely
were used. Once oysters began to spawn, they were rinsed and continued to spawn in separate containers. Prior to fertilisation, all eggs were pooled separately for each mass
spawning, and this process was repeated for all the sperm samples. The complete spawning procedure outlined previously was repeated for the spawning of subsequent
generations. Control, or nonselected oysters, were taken from the same four estuaries as the original base population. An increase in whole weight of 18 was gained by the
Ž .
third generation Nell et al., 1999 . Production of triploid Sydney rock oysters rates has also been examined for its potential for increasing whole weight and it was found that
triploid oysters were on average 41 heavier than their diploid counterparts after 2.5 Ž
. years of growth Nell et al., 1994 .
Selective breeding programs may result in the loss of genetic variation through Ž
. inadequate numbers of parents, leading to inbreeding Tave, 1993 . Farming has led to
Ž .
the reduction of genetic diversity in Pacific oysters C. gigas in the United States of Ž
. Ž
. America Hedgecock and Sly, 1990 and in United Kingdom Gosling, 1982 . Hedge-
Ž .
cock and Sly 1990 also found that the effective population sizes of the U.S. stocks
were several orders of magnitude smaller than the number of broodstock actually used by industry. In Tasmania, farming has not led to the loss of appreciable amounts of
Ž .
genetic variation English et al., 2000 . A previous study of Sydney rock oysters from three sites in New South Wales found
Ž .
heterozygosities ranging from 0.17 to 0.19 Buroker et al., 1979a . Ours is the first study to examine the genetic variability in a selected line. Allozyme electrophoresis was used
to determine the levels of genetic diversity of the second- and third-generation oysters and of a control, unselected, line.
2. Methods
