1. Introduction
Ž .
Ž The brown trout Salmo trutta L. occurs in two geographical forms in France Krieg
. and Guyomard, 1985 , the Mediterranean form and the Atlantic form, which are
Ž assumed to have diverged in allopatry almost 50,000 years ago estimated from mtDNA
. data in Bernatchez et al., 1992 and Giuffra et al., 1996 . Their Nei’s genetic distance
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based on allozymic data is high, averaging 0.10 Guyomard, 1989 . Their coexistence in rivers of the Mediterranean basin is generally artificial and caused by secondary contact
through human stocking with domesticated strains of Atlantic origin. Ž
Several protein-coding loci discriminate between the two forms Guyomard, 1989; .
Poteaux et al., 1998 . It is, therefore, possible to estimate the genetic impact of Ž
domesticated introductions. Studies of stocked populations of brown trout Krieg and Guyomard, 1985; Barbat-Leterrier et al., 1989; Largiader et al., 1996; Poteaux and
. Berrebi, 1997 have shown that hybrids, recombined for two or more generations, are
frequent in rivers. Although F hybrids display good survival and fecundity under
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Ž .
controlled conditions Guyomard, 1997; Mezerra, unpublished data , introgression is a limited process, in natura, and is often associated with deviation from Hardy–Weinberg
Ž expectations and linkage disequilibria
Barbat-Leterrier et al., 1989; Poteaux and .
Berrebi, 1997 . One of the hypotheses put forward to account for the low levels of introgression and
the numerous disequilibria observed in natural populations involves selective pressures Ž
. acting against F
or recombined individuals F
or back-crosses . These selective
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Ž . processes may be caused by two types of genetic mechanism: i additive selection,
acting against several independent loci; in this case, some genes of domesticated strains Ž .
may be unsuited to local conditions and tend to be eliminated in rivers; or ii selection acting on epistatic interactions among genes or on complexes of co-adapted genes set up
during the divergence time of the taxa. Such selection would tend to prevent the disruption of these co-adapted complexes which may occur during hybridisation.
Ž .
Ž .
According to Hunt and Selander 1973 and Mayr 1974 , the existence of such groups of co-adapted complexes may have acted as an effective brake on introgression, even in
the absence of a behavioural barrier. The high mutation rate of microsatellites, and their expected selective neutrality are
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favourable characters for pedigree and linkage studies Hearne et al., 1992 . The number of available loci is also high: in brown trout, rough estimates of the number of the motifs
Ž .
Ž .
Ž .
GT and CT
give a total of about 150,000 Estoup et al., 1993 . As a consequence
n n
of their high heterozygosity level and large genetic differences between individuals, the number of informative parents is larger than with protein-coding loci, which display a
much lower polymorphism level. We performed a back-cross experiment using F individuals to test the hypothesis of
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possible genomic incompatibility between the two forms. Here, we present preliminary results of a larger experiment designed to identify chromosomal segment, which could
exhibit incompatibility at meiosis in F between the two brown trout subspecies. Parents
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and their offspring were genotyped with four microsatellite and five allozymic loci. Mendelian single and joint segregation of these loci were tested in the offspring of
female hybrids. Significant tests were observed. Segregation that displayed statistically
significant deviation from expected proportions is discussed with respect to linkage or selection.
2. Material and methods
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