The Journal of Heredity 2001:92(2)
© 2001 The American Genetic Association 92:146-149
Alternative Mating Strategies in Atlantic Salmon and Brown Trout
From the Departamento de Biologia Funcional, Universidad de Oviedo, C/ Julian Claveria s/n, 33006-Oviedo, Spain (Garcia-Vazquez, Martinez, and Perez), Departamento de Bioquimica, Genetica e Inmunologia, Universidad de Vigo, Facultad de Ciencias, Vigo, Spain (Moran), and Ecologie des Poissons, INRA Station d'Hydrobiologie, Saint Pée sur Nivelle, France (de Gandemar and Beall).
Address correspondence to Eva Garcia-Vazquez at the address above or e-mail: egv{at}sauron.quimica.uniovi.es.
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Abstract |
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By screening variable number of tandem repeat (VNTR) loci, multiple paternity within clutches has been found in wild populations of southern European Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). For Atlantic salmon, we determined the relative contribution of alternative male phenotypes to the next generation. Individual males that are morphologically juvenile yet sexually mature fertilized a large proportion of eggs, and they thereby contributed to an increase of genetic variability in wild populations via (1) balancing the sex ratio, (2) increasing outbreeding, and (3) enlarging the effective population size, in part a consequence of (1) and (2). In addition, these precocious males ensured that interspecific spawns involving Atlantic salmon females and brown trout males (a fairly common occurrence in southern Europe where the two species are sympatric) resulted mostly in Atlantic salmon progeny. For brown trout, preliminary genetic results indicated that multiple paternity, when present, was not due to alternative mating strategies by males, but rather to successive fertilizations by adult suitors.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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Following the introduction of molecular genetic markers to the study of natural populations and animal behavior, new experiments could be envisioned to critically test earlier observations and resolve long-standing hypotheses in molecular ecology (Avise 1994; Carvalho 1998). As an example, the existence of multiple paternity in fish had been proposed and demonstrated in many species (see Arnold 1996), but until the advent of molecular approaches the extent of this reproductive behavior could not be accurately quantified.
We have studied the reproductive behavior in both Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) for the last 10 years. The Atlantic salmon, in particular, is an interesting species due to the fact that the males may present two alternative (and compatible) reproductive phases: an anadromous tactic, wherein after a growing phase in the sea the salmon then return to a river as fertile adults; or alternatively a precocial tactic wherein males can reach maturity at 1–3 years of age and perhaps reproduce in a river before migrating to the sea (Meerburg 1986). Various experiments have been carried out by us (Martinez et al. 2000; Moran and Garcia-Vazquez 1998; Moran et al. 1996) and other researchers (Hutchings and Myers 1988; Jordan and Youngson 1992; L'Abbée-Lund 1989; Thomaz et al. 1997) to test the reproductive consequences of these different lifestyles. All authors agree in the capability of "mature parr" (also known as "'ecocious parr," or "juvenile mature males") to fertilize female eggs, and in the importance of their contribution to genetic variability in natural populations. This fact is especially important in small rivers and in declining populations, where an increase in the effective number of parents is essential from the point of view of conservation genetics. However, other aspects of the contribution of juvenile mature males to the spawning process in salmon have not been fully explored. For example, what is the impact of mature parr on the sex ratio of wild Atlantic salmon populations, and what is the extent of their involvement in the breeding scheme compared to that of anadromous adult males?
Interspecific hybridization between Atlantic salmon and brown trout has also been widely studied (see Verspoor and Hammar 1991), and both indirect (Elo et al. 1995; Jansson and Ost 1997) and direct (Gephard et al. 2000) evidence suggests that the reproductive behavior of mature male parr should play an important role in the process. These males may affect hybridization in two opposite ways. On the one hand, they could increase the hybridization rate if they participate as sneakers in brown trout spawnings (as they do in intraspecific crosses of adult Atlantic salmon). On the other hand, they might diminish between-species hybridization if they compete with brown trout males when only the latter are otherwise in the presence of mature Atlantic salmon females. The ability of precocious parr to stimulate Atlantic salmon females and fertilize their eggs in the absence of adult salmon males has been demonstrated in experimental conditions and also in the wild (Martinez et al. 2001).
In the present work, we extend our investigations of salmonid reproduction by performing new sets of experiments and a more detailed analysis of the implications of alternative male reproductive forms. One goal is to compare the reproductive strategies of Atlantic salmon and brown trout. Another goal is to assess the possible roles of precocious male parr in maintaining genetic variability in wild populations of Atlantic salmon, in altering the sex ratio, in enhancing outbreeding, and in influencing the extent of between-species hybridization. We show that competition among adult males is a primary mechanism responsible for multiple paternity in brown trout.
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Materials and Methods |
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Fish Samples and Experimental Procedures
All of the experiments were conducted in the Lapitxuri stream (south France) and in an experimental channel diverted from this stream (description in Beall et al. 1997). Adults and juveniles caught by electrofishing were anaesthetized and a bit of adipose fin was clipped and ethanol preserved for genetic analysis. Embryos were sampled directly from the redds and ethanol preserved. The contribution of both types of males (anadromous and mature juveniles) to adult female spawning was determined as explained in Martinez et al. (2000), employing nine variable number of tandem repeat (VNTR) loci as genetic markers. These hypervariable markers allowed us to unequivocally identify the parents (mother and father) of every embryo analyzed.
Field surveys of Atlantic salmon adults were carried out in the Spanish rivers of Esva, Narcea, Sella, and Cares, and in the French river Nivelle. Fish taken by electrofishing and angling were sampled during the prespawning (spring, summer) and spawning (autumn) seasons in 1997. Samples of blood were drawn using sterile syringes. Fin samples were also taken and scales were sampled for age determination.
Field surveys to estimate interspecific hybridization rates were carried out by electrofishing. Juveniles and adults of both Atlantic salmon and brown trout were sampled from seven southern European rivers, cited from west to east: Eo, Porcia, Esva, Narcea, and Cares rivers in Spain, and the Nivelle and Nive rivers in France.
Brown trout embryos were sampled from wild redds after using a VHR camera to observe and record the courtship behavior of adults in selected areas of the River Sella.
Sex and Age Determination
The ages of individual fish were determined by scale analysis as described by Bagliniere (1985). An individual's sex was determined by an immunoagglutination assay, which tests for serum vitellogenin that is present in the blood of females (Le Bail and Breton 1981).
DNA Analyses
Total DNA was isolated from muscle tissue or adipose fin following the phenol-chloroform method described by Taggart et al. (1992). For microsatellite analysis, we employed a Chelex extraction technique (Estoup et al. 1996).
VNTR loci were used for individual identification and parentage analysis. Minisatellite loci pSa-45/1, pSsa45/2, pSsa-A60, pStr-A22/1, and pStr-A5 (kindly provided by J. Taggart and P. Prod;auohl) were employed as described in Perez et al. (1997). The microsatellites studied were SSOSL417 (Slettan et al. 1995), SS3 (Martinez et al. 1999b), and SS4 and SS6 (Martinez et al. 1999a). Conditions of polymerase chain reaction (PCR) amplification, electrophoresis, and gel staining, and methods of parentage assignment are described in Martinez et al. (2000).
Hybrid Identification
Methods of hybrid identification, following Verspoor (1988), were based on protein electrophoresis of two muscle-expressed enzyme loci, GPI-3 and PGM-2, which encode phosphoglucose isomerase and phosphoglucomutase. Amplification of the 5S RNA coding gene and the spacer was also employed, as described in Pendas et al. (1995). Mitochondrial DNA analysis was used to identify the mother species in the hybrids (see Gephard et al. 2000).
Effective number of Breeders
Estimates of the effective number of breeders, Nb, were made employing the expression
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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Reproduction by Alternative Male Phenotypes in Atlantic Salmon
Table 1 presents a summary of the paternity contribution of each male reproductive type to the collection of offspring within the redds of four different Atlantic salmon females. The relative contribution of the precocious parr varied greatly, ranging from 42% of the embryos in the two redds of female A, to 86% of the embryos of the single redd of female B. Overall these mature juveniles sired 65% of the 126 embryos sampled from these four females, a value that contrasts with those obtained for Atlantic salmon by some other authors. For example, Jordan and Youngson (1992) reported that only about 20% of the salmon embryos in a Scottish river were sired by juvenile males. However, our results for the Nivelle River population agree with those obtained in semiartificial conditions in the same stream. Perhaps the contrasting outcomes in different rivers somehow reflect differences in juvenile maturation rates expected at different latitudes (43°N, southern France, versus 57°N, Scotland). Growth rates during the spring are higher in lower latitudes and this might translate into a greater likelihood of paternity for precocious males in southern rivers (Prevost et al. 1992; Rowe and Thorpe 1990). In any case, we conservatively estimate that mature juvenile Atlantic salmon contribute the majority of offspring to the next generation in the Nivelle River.
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The huge contribution of precocious parr to Atlantic salmon reproduction in southern European populations carries important implications with respect to sex ratio, effective number of breeders, and between-cohort mating rate. For example, Table 2 presents estimates for each of these three parameters for salmon populations in five rivers entering the Bay of Biscay (latitude 43°N), assuming two widely different levels of reproductive contribution by precocious parr (0% versus the observed 60%). First, these contrasts clearly show that mature juvenile males compensate for what would otherwise be a highly unbalanced adult sex ratio. This could be very important because a balanced sex ratio might avoid egg wastage due to a shortage of adult males, a common situation at the end of the spawning season because many adult males die or are exhausted. Moran et al. (1996) found that in cases of overmaturation or exhaustion of adult males, the reproductive contribution of mature juveniles increases significantly, and that these precocious parr can even take over the fertilization of eggs in the absence of adult males (Martinez et al. 2001).
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Second, Table 2 shows that sexually mature juveniles generate a considerable increase in the number of breeders (Nb) in a local population. L'Abbée-Lund (1989) emphasized the relevance of mature juveniles for the conservation of small Atlantic salmon populations, as did Martinez et al. (2000). It is probable that the high maturation rate of juvenile salmon in southern Europe has contributed to the survival of their populations in recent decades, even when the number of returning anadromous adults has decreased dramatically (Bra;ña 1995). Finally, Table 2 shows a high between-cohort mating rate (due to the participation of mature juvenile males in the spawning process). This no doubt reduces the probability of inbred crosses, and thereby serves to avoid losses of genetic variability in small populations (as proposed by Moran and Garcia-Vazquez 1998).
Mature juvenile males may have saved south European Atlantic populations from extinction, given the depressed size of populations for a number of decades. This point suggests that these populations may be under intense selection for maturation of juvenile males, and hence that the relative preponderance of mature juvenile males in the southern populations may be an adaptive response to anthropogenic depletion of anadromous salmon numbers.
Reproduction by Male Brown Trout
A second objective of our work is to compare the two Salmo species with respect to alternative male mating strategies. For brown trout, we recorded the spawning activity of this species in the River Sella and conducted further paternity analyses in some of the observed redds. In this preliminary survey, multiple paternity usually was detected in wild redds (Table 3): in three of four redds, more than one male fertilized the female's eggs. In each case the pattern was similar: one male fertilized most eggs (63–100%), and one or more additional males fertilized the remainder of the clutch. From our records and direct observations, multiple paternity in brown trout seems to be due to consecutive adult males who court the adult female (rather than to any sneaking behavior by males). Thus the reproductive strategy of this species seems to be different from that of Atlantic salmon.
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The life cycle of the brown trout in southern European rivers can be anadromous and/or sedentary. During their first year, sedentary individuals can mature in freshwater as small but normal adults, and the breeding populations are generally composed of females and males from more than four different age cohorts (Garcia and Braña 1988; Lobon-Cervia et al. 1986). The main reproductive difference between brown trout and Atlantic salmon in southern European rivers is the lack of evidence for sneaking behavior in small maturing brown trout. Perhaps the precocious sneaker parr in Atlantic salmon are a first step in an evolutionary change from anadromy to diadromy.
Interspecific Hybridization
Table 4 presents observed rates of interspecific hybridization between Atlantic salmon and brown trout in seven southern European rivers. These values, which apply to juveniles and adults rather than embryos, indicated that on average about 3.4% of a population consists of hybrids. In all cases, the maternal species in a cross was Atlantic salmon. Thus some "loss" of Atlantic salmon female gametes is expected due to this not-so-rare phenomenon of interspecific mating.
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In experimental settings where brown trout males and precocious salmon parr are placed in competition for access to salmon females, the juvenile salmon males are also "sneakers" in the interspecific matings (Table 5). In these types of experiments, eggs are fertilized by mature salmon juveniles in proportions similar to those found in intraspecific Atlantic salmon crosses in the wild at the same latitude, that is, 65% (Martinez et al. 2000). Thus not all the offspring from Atlantic salmon females courted by brown trout males are hybrids; an important proportion consists of Atlantic salmon.
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In conclusion, precocious Atlantic salmon parr contribute to balance the sex ratio, enlarge the effective population size, and increase outbreeding. In addition, they fertilize most eggs in the interspecific matings between Atlantic salmon and brown trout. Sneaking behavior has not been evidenced in small maturing brown trout, this being the main reproductive difference between brown trout and Atlantic salmon in wild southern European populations.
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Acknowledgments |
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This work was financially supported by the Spanish DGICYT (HF 1997-0215 and PB98-1570) and by the French INRA-Région d'Aquitaine (contract 940308004 and Integrated Action 93187). We are indebted to John Avise, who very kindly revised the manuscript. We are grateful to the Regional Asturian Administration (Servicio de Caza y Pesca Fluvial) from Spain, and to the Nivelle Fishermen's Association and INRA staff from France for collaborating in sampling tasks.
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Footnotes |
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This paper was delivered at a symposium entitled "DNA-Based Profiling of Mating Systems and Reproductive Behaviors in Poikilothermic Vertebrates" sponsored by the American Genetic Association at Yale University, New Haven, CT, USA, June 17–20, 2000.
Corresponding Editor: John C. Avise
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