Variation in development rate among clonal lines of rainbow trout (Oncorhynchus mykiss)

Abstract

The genetic basis of development rate was examined using clonal lines of rainbow trout produced by androgenesis. We examined development rate, as indicated by physiological time to hatch (°days), of four clonal lines of rainbow trout crossed to outbred females in three separate experiments. In the first experiment, the Swanson line, derived from an Alaskan population, displayed a significantly earlier physiological time to hatch than the Arlee line, derived from a domesticated rainbow trout (P<0.001). In the second experiment, this trend was maintained across three temperatures (approximately 8, 10, and 14°C) and two females, although the experiment lacked the statistical power to detect a difference (P=0.052). In the third experiment, the Swanson line had a significantly earlier physiological time to hatch than three other clonal lines (P<0.01) when tested across three temperatures (8, 10, and 16°C) and two females. There was no evidence of genotype by environment interaction in any of the experiments. These data indicate that development rate has a strong genetic component, and that the clonal lines of rainbow trout available should be highly suitable for QTL analysis of development rate in rainbow trout.

Introduction

Development rate is an important trait which figures prominently in both the evolutionary history and the successful culture of rainbow trout (Oncorhynchus mykiss). Differences in developmental timing are considered to be critical in bringing about evolutionary change (Gould, 1981), and differences in development rate among strains have been identified in rainbow trout and other salmonids (Ferguson et al., 1985; Tallman, 1986). In rainbow trout, heritability of development rate has been estimated as high as 0.23 (McIntyre and Blanc, 1973). Rate of development in salmonids is important in synchronizing time of emergence of fry (Brannon, 1987), which can reduce predation (Godin, 1982) or maximize food availability. In the culture environment, knowledge of the rate of development is often desirable, as this allows the culturist more flexibility in allocating space for newly hatched fry. In addition, strains of quickly developing trout might be desirable, as fast development has been associated with fewer developmental deformities, larger size, and earlier sexual maturity in some instances (Allendorf et al., 1983).

Despite the importance of development rate to the evolutionary history and successful aquaculture of rainbow trout, the genetic basis of development rate is largely unknown. Allendorf et al. (1983)showed that a regulatory mutation in the expression of liver phosphoglucomutase could alter development rate in one rainbow trout strain. However, many questions about the genetic basis of development rate remain. How many genes control the rate of development? What are their relative effects? What implications does this have for natural selection and life history evolution? How can we use this information to improve cultured stocks of rainbow trout?

Questions such as these can be addressed using clonal lines of rainbow trout. The high fecundity, external fertilization, ease of culture, and tolerance of chromosomal manipulations exhibited by rainbow trout have allowed the production of clonal lines for use in experimental research, analogous to the inbred strains of mice that currently play a central role in biomedical research (Silver, 1995). Clonal lines of trout are produced by androgenesis, in which the egg is irradiated with gamma radiation prior to fertilization, and the first cleavage is prevented with heat or pressure shock (Parsons and Thorgaard, 1985). The resultant fish are homozygous, and a second round of androgenesis or gynogenesis is then performed. This results in a family of fish which are completely homozygous and genetically identical to one another (Young et al., 1996). The fish are all male (YY) or all female (XX), and strain perpetuation is accomplished through androgenesis or gynogenesis, respectively. Clonal lines have already been used in the development of a genetic linkage map for rainbow trout (Young et al., 1998). The clonal lines of fish produced in this manner are also good candidates for genetic analysis of quantitative traits such as development rate.

To facilitate genetic analysis of development rate, however, lines which display genetic variation in this trait must be identified. One approach for characterizing clonal lines of interest is to cross the lines to outbred rainbow trout eggs, and measure the development rate of the hybrid progeny.

The objectives of this study, therefore, are to 1) determine if crossing strains to outbred eggs is a powerful enough technique to detect differences in quantitative traits, and 2) to identify clonal lines of rainbow trout which differ significantly in development rate, indicating their suitability for subsequent genetic analysis.