A lethal effect associated with polymorphism of the NOR-bearing chromosomes in rainbow trout (Oncorhynchus mykiss)

Abstract

Cytogenetic analysis of a rainbow trout stock showed that the nucleolar organizing regions were located subterminally on the long arm of a submetacentric chromosome pair and occurred as a single chromosomal segment (phenotype N1) or as two chromosomal segments separated by a short euchromatic segment (phenotype N2). Cytogenetic analysis also showed that there were N1N1 and N1N2 individuals but no N2N2 individuals. Analysis of the different includedphenotypes incluted that the population was not in Hardy-Weinberg equilibrium (χ 2 = 19.333; p < 0.01), and that a higher frequency of individuals had the N1N2 phenotype. Experimental crosses involving four males (two N1N1 and two N1N2) and four females (one N1N1 and three N1N2) yielded eight broods. There were no significant differences between the expected and observed frequencies of offspring resulting from crosses involving N1N1 x N1N2 individuals. However, significant differences were seen in crosses involving N1N2 x N1N2 parents because of the a high incidence of N1N2 fishes and the absense of N2N2. The lack of N2N2 individuals in the parental sample and their absence among the offspring of the experimental crosses suggested that this genetic combination may be lethal in rainbow trout. The survival rates of embryonic "eyed egg" and fry stage individuals were not different, indicating that the possible lethal effect may occur during more advanced ontogenetic phases.

Chromosome polymorphism of heterochromatin and nucleolar regions in two populations of the fish Astyanax bockmanni (Teleostei: Characiformes)

“…A pericentric inversion of the first karyotype pair was detected in Serrasalmus spilopleura but without the structural homozygote (Centofante et al, 2002). Although this may be due to a sampling bias, such a situation seems to be comparable to our results where the homozygotes for the chromosomes with inversions were not found, suggesting that homozygosity might be a lethal genotype similar to the situation in O. mykiss (Porto-Foresti et al, 2004). Moreover, according to Mariotto et al (2009), in the species Ancistrus cuiabae it would be possible to correlate differential fitness with distinct frequencies of cytotypes bearing chromosome inversions.…”

“…Chromosome rearrangements due to inversions have been described in some fish groups (Centofante et al, 2002;Jorge & Moreira-Filho, 2004), with some cases involving the NORbearing chromosomes (Porto-Foresti et al, 2004;Mariotto et al, 2009). Based on experimental crosses of farmed stocks of the salmonid Oncorhynchus mykiss, Porto-Foresti et al (2004) suggested that a particular combination of chromosomes with inversions might be lethal in rainbow trout. For natural populations of neotropical fish, there are few descriptions of the biological effects related to this kind of chromosome polymorphism.…”

Cytogenetic analysis and chromosomal characteristics of the polymorphic 18S rDNA in the fish Prochilodus lineatus (Characiformes, Prochilodontidae)

“…Size polymorphisms of the NORs are relatively common in Neotropical fishes (Foresti et al, 1981;Brum et al, 1998;Vicari et al, 2005) and are sometimes also associated with sex chromosomes (Galetti Jr., 1998;Molina et al, 1998;Born andBertollo, 2000, Artoni andBertollo, 2002;Vicari et al, 2003). Interestingly, a lethal effect in the rainbow trout (Oncorhynchus mykis) has been found to be related to a NORs size polymorphism in homozygotes, with evidence 622 Vicari et al also showing that heterozygotes present a higher adaptive value (Porto-Foresti et al, 2004). In piscines, the involvement of heterochromatin in the accumulation of rDNA loci through unequal crossing-over or sister chromatid exchanges involving repeated sequences and adjacent loci has been proposed as the main mechanism for these rDNA polymorphisms (Pendás et al 1993).…”

Chromosome polymorphism in Ancistrus cuiabae Knaack, 1999 (Siluriformes: Loricariidae: Ancistrini)

“…Porto- Foresti et al (2004), detected a NOR polymorphism in Oncorhynchus mykiss from controlled breeding, suggesting that heterozygote individuals would have a greater adaptive value in relation to the homozygote ones. Even though the reduced frequency of cytotype C may be due to a sampling bias, it is tempting to correlate the distinct frequencies of the A. cuiabae cytotypes with a differential fitness, as suggested for O. mykiss.…”