Genetic basis of differences in myxospore count between whirling disease-resistant and -susceptible strains of rainbow trout

Abstract

We used a quantitative genetics approach and estimated broad sense heritability (h 2 b ) of myxospore count and the number of genes involved in myxospore formation to gain a better understanding of how resistance to Myxobolus cerebralis, the parasite responsible for whirling disease, is inherited in rainbow trout Oncorhynchus mykiss. An M. cerebralis-resistant strain of rainbow trout, the German Rainbow (GR), and a wild, susceptible strain of rainbow trout, the Colorado River Rainbow (CRR), were spawned to create 3 intermediate crossed populations (an F1 cross, F2 intercross, and a B2 backcross between the F1 and the CRR). Within each strain or cross, Comparison of means and a joint-scaling test suggest that resistance alleles arising from the GR strain are dominant to susceptible alleles from the CRR strain. Resistance was retained in the intermediate crosses but decreased as filial generation number increased (F2) or backcrossing occurred (B2). The estimated number of segregating loci responsible for differences in myxospore count in the parental strains was 9 ± 5. Our results indicate that resistance to M. cerebralis is a heritable trait within these populations and would respond to either artificial selection in hatcheries or natural selection in the wild. KEY WORDS: Myxobolus cerebralis · Oncorhynchus mykiss · Parasite · Salmonid · Heritability · Quantitative genetics Resale or republication not permitted without written consent of the publisherDis Aquat Org 102: [97][98][99][100][101][102][103][104][105][106] 2012 process presumably led to the production of hatchery-derived rainbow trout strains that are resistant to Myxobolus cerebralis. El-Matbouli et al. (2002) found that, under experimental laboratory conditions, a German strain of rainbow trout (German Rainbow, GR) was at least as resistant to M. cerebralis as brown trout Salmo trutta, which evolved with the parasite in its European home range (Hoffman 1970). The GR strain was also found to be more resistant to M. cerebralis than either the North American Trout Lodge (TL) or Colorado River Rainbow (CRR) trout strains (Hedrick et al. 2003, Schisler et al. 2006. Resistance likely arose through the growth and reproduction of the GR strain under continuous exposure to the parasite in Bavarian hatcheries (Hedrick et al. 2003).Resistance to disease in animals is often a complex, polygenic trait (Grenfell & Dobson 1995) that results from a series of complex interactions among the host, pathogen, and environment (Snieszko 1974, Hedrick 1998. The mechanisms for resistance to Myxobolus cerebralis seen in the GR strain, like those seen in trout resistant to a similar myxosporean, Ceratomyxa shasta, are suspected to be polygenic (Hedrick et al. 2001). Studies examining differential gene expression in resistant and susceptible strains of rainbow trout have identified several genes potentially involved in resistance (Severin & El-Matbouli 2007, Baerwald et al. 2008, Severin et al. 2010. Baerwald et al. (2011) discovered a major quantitative trait lo...

Survival and Reproduction of Myxobolus cerebralis-Resistant Rainbow Trout Introduced to the Colorado River and Increased Resistance of Age-0 Progeny

“…Age-0 CRR had a two-fold and 127-fold higher myxospore count, on average, than either the brown trout or GR-cross fry, respectively, and this is consistent with other studies showing that CRR are highly susceptible to M. cerebralis infection [9], [12], [40]. Myxospore levels in CRR individuals indicate that the parasite is still prevalent in the upper Colorado River and that the low myxospore levels in the GR-cross are not a result of reduced parasite numbers.…”

“…The German Rainbow (GR) is a hatchery-derived rainbow trout strain that was exposed to M. cerebralis for decades in a hatchery in Germany [11]. Although the GR strain can be infected with M. cerebralis , parasite burdens are usually low [9], [11], [12] and the GR strain is known to survive and reproduce in the presence of, and when infected with, M. cerebralis . Low parasite burdens and the strain’s ability to persist when exposed to M. cerebralis have been termed “resistance,” and this resistance is presumed to be a result of long-term exposure to the parasite over multiple generations [11].…”

“…Despite the resistance seen in the GR strain, its survival and viability in the wild was uncertain due to the strain’s history of domestication [9]. Therefore, the GR strain was experimentally crossed with the Colorado River Rainbow (CRR) [9], [12], [13], a wild rainbow trout strain that had been widely stocked in Colorado and was used to establish many naturally reproducing wild rainbow trout fisheries prior to the introduction of M. cerebralis [14].…”

“…Laboratory experiments showed that the first filial (F 1 ) generational cross between the two strains (termed the GR×CRR) exhibited resistance characteristics similar to those of the GR strain [9], [12], and was capable of attaining critical swimming velocities similar to those of the CRR strain [13]. It was suggested that the GR×CRR cross may be the best candidate for reintroducing rainbow trout populations; however, its utility needed to be evaluated in a natural setting [12]. Overall, we wanted to evaluate the performance of GR×CRR that were stocked into the upper Colorado River in an attempt to reintroduce a self-sustaining population in the presence of M. cerebralis .…”

Behavioral responses of a highly domesticated, predator naïve rainbow trout to chemical cues of predation

“…This strain is fast-growing, matures early (Schisler and Fetherman, 2009), and has a low parasite burden following exposure to M. cerebralis (Fetherman et al, 2012;Hedrick et al, 2003;Schisler et al, 2006), making it desirable in rearing and stocking programs. However, due to its history of domestication, survival in the wild has been low (Fetherman and Schisler, 2013), and it is particularly susceptible to predation post stocking relative to other strains (unpublished data).…”