94
M.S. Clark et al.
aquaculture crop. Using the new MPSS (Massive Parallel Signature Sequencing)
technology, Hedgecock et al. (2007) studied the genetic and physiological causes of
heterosis (hybrid vigor) and its converse, inbreeding depression in the Pacific oyster,
Crassostrea gigas, the causes of which have remained elusive for nearly a century
(e.g. Crow 1998). Explanations proposed for heterosis have included:
• Overdominance (the superiority of heterozygotes at genes affecting fitness traits)
• Dominance (the masking of deleterious recessive mutations in hybrids by
dominant alleles inherited from one or the other inbred parent)
• Epistasis (the interaction of alleles at different loci).
In previous investigations, the evidence for heterosis in bivalves was indirect. An
observed correlation between a heterozygous marker and fitness-related traits, such
as growth was established (e.g. Hedgecock et al. 1995). Also controlled crosses
including the study of F2 hybrid populations revealed a high load of deleterious recessive mutations concordant with the dominance hypothesis (Launey and
Hedgecock 2001).
The physiological causes of growth heterosis are far less studied than the genetic
origins, but it was shown that both larval and adult hybrid oysters have higher feeding rates and efficiencies than their inbred counterparts (Bayne et al. 1999, Pace
et al. 2006). The two main aims of the Hedgecock et al. (2007) study were:
• To estimate the number of genes implicated in the hybrid vigour of oyster
• To provide a genome-wide scan for the causes for heterosis.
Gene-expression patterns underlying growth heterosis were analysed in two partially inbred (f = 0.375) and two hybrid larval populations produced by a reciprocal
cross between the two inbred families. cDNAs were cloned and 4.5 Mb of sequence
tags were generated. The sequences contained 23,274 distinct signatures (i.e. short
read sequence tags) that were expressed at non-zero levels, and showed a highly
positively skewed distribution with median and modal counts of 9.25 million and
three transcripts per million, respectively. For nearly half (57%) of these signatures,
expression levels were shown to depend on genotype. Results demonstrated that
this phenomenon is predominantly non-additive (hybrids deviate from the inbred
average as in the over- or underdominance hypotheses), and that overdominance
was prevalent in explaining expression patterns in the Pacific oyster as opposed to
results reported in maize or Drosophila (Swanson-Wagner et al. 2006, Gibson et al.
2004).
The genetic basis of overdominant phenotypes may be due to interactions
between cis-acting regulatory elements or differences in levels of trans-acting factors. As noted by Hedgecock et al. (2007), it should be possible to distinguish
between cis- and trans- regulation of expression levels by contingency tests on the
linkage between expression and genotype in the next generation:
M.S. Clark et al.
aquaculture crop. Using the new MPSS (Massive Parallel Signature Sequencing)
technology, Hedgecock et al. (2007) studied the genetic and physiological causes of
heterosis (hybrid vigor) and its converse, inbreeding depression in the Pacific oyster,
Crassostrea gigas, the causes of which have remained elusive for nearly a century
(e.g. Crow 1998). Explanations proposed for heterosis have included:
• Overdominance (the superiority of heterozygotes at genes affecting fitness traits)
• Dominance (the masking of deleterious recessive mutations in hybrids by
dominant alleles inherited from one or the other inbred parent)
• Epistasis (the interaction of alleles at different loci).
In previous investigations, the evidence for heterosis in bivalves was indirect. An
observed correlation between a heterozygous marker and fitness-related traits, such
as growth was established (e.g. Hedgecock et al. 1995). Also controlled crosses
including the study of F2 hybrid populations revealed a high load of deleterious recessive mutations concordant with the dominance hypothesis (Launey and
Hedgecock 2001).
The physiological causes of growth heterosis are far less studied than the genetic
origins, but it was shown that both larval and adult hybrid oysters have higher feeding rates and efficiencies than their inbred counterparts (Bayne et al. 1999, Pace
et al. 2006). The two main aims of the Hedgecock et al. (2007) study were:
• To estimate the number of genes implicated in the hybrid vigour of oyster
• To provide a genome-wide scan for the causes for heterosis.
Gene-expression patterns underlying growth heterosis were analysed in two partially inbred (f = 0.375) and two hybrid larval populations produced by a reciprocal
cross between the two inbred families. cDNAs were cloned and 4.5 Mb of sequence
tags were generated. The sequences contained 23,274 distinct signatures (i.e. short
read sequence tags) that were expressed at non-zero levels, and showed a highly
positively skewed distribution with median and modal counts of 9.25 million and
three transcripts per million, respectively. For nearly half (57%) of these signatures,
expression levels were shown to depend on genotype. Results demonstrated that
this phenomenon is predominantly non-additive (hybrids deviate from the inbred
average as in the over- or underdominance hypotheses), and that overdominance
was prevalent in explaining expression patterns in the Pacific oyster as opposed to
results reported in maize or Drosophila (Swanson-Wagner et al. 2006, Gibson et al.
2004).
The genetic basis of overdominant phenotypes may be due to interactions
between cis-acting regulatory elements or differences in levels of trans-acting factors. As noted by Hedgecock et al. (2007), it should be possible to distinguish
between cis- and trans- regulation of expression levels by contingency tests on the
linkage between expression and genotype in the next generation: