Donald C. Morizot

Gene Nomenclature for Protein‐Coding Loci in Fish

Abstract The Fish Genetics Section of the American Fisheries Society established its Nomenclature Committee to develop and promote standardized genetic nomenclatures. Here, following public comments on previously published draft guidelines, we present the committees revised version of a nomenclature for protein-coding loci in fish. This nomenclature closely parallels the one used for human genetics, but improves on it in several respects. The fish system (1) includes standardized abbreviations for commonly analyzed proteins, and provides formal symbols for gene loci encoding these proteins; (2) specifies typographic conventions for distinguishing between genes and proteins and for identifying alleles; (3) provides for multilocus isozyme systems, isoloci, regulatory loci, and pseudogenes; (4) allows important basic information (such as subcellular distributions of gene products, active substrate isomers, recent gene duplicates, and orthologous relationships among loci) to be specified in gene symbols via ...

Dissolution of Sexual Signal Complexes in a Hybrid Zone between the Swordtails Xiphophorus birchmanni and Xiphophorus malinche (Poeciliidae)

Abstract The evolution of sexual signaling systems is influenced by natural and sexual selection acting on complex interactions among traits. Natural hybrid zones are excellent systems for assessing fitness effects on sexual phenotypes. Most documented hybrid zones, however, show little variation in sexual signals. A hybrid zone between the swordtails Xiphophorus birchmanni and Xiphophorus malinche is characterized by numerous recombinants for male sexual traits. Analyses of geographic variation in morphological and isozyme traits in the Río Calnali, Hidalgo, Mexico, reveal an upstream-to-downstream gradient from X. malinche- to X. birchmanni-type traits. A second hybrid zone, likely isolated from the R. Calnali, occurs in the nearby Arroyo Pochutla. Although the presumed female preference for swords predicts the introgression of swords from X. malinche-like populations into hybrid populations, the opposite pattern was observed. Swords are reduced in populations otherwise characterized by X. malinche traits. Sexually dimorphic traits were poorly correlated within individuals, indicating that sexual selection does not act against recombinant phenotypes. Hybrid males also exhibit trait values outside the range of parental variation. These patterns are consistent with predictions that females are permissive, preferring generally conspicuous males without attending to specific features.

Localization of a CDKN2 gene in linkage group V of Xiphophorus fishes defines it as a candidate for the DIFF tumor suppressor

The Xiphophorus hybrid melanoma model represents one of the earliest reported cases of genetically regulated tumor susceptibility. Melanoma formation in Xiphophorus hybrids may be explained by the inheritance of two genes: a sex‐linked oncogene, Xmrk, and a putative tumor suppressor locus, termed DIFF, located in Linkage Group V (LG V). Several genetic mapping procedures were used to produce a new Xiphophorus LG V map with 20 loci. All markers, particularly a recently cloned Xiphophorus CDKN2 gene family member, called CDKN2X, were tested for associations of genotype with degree of macromelanophore pigment pattern modification and susceptibility to melanoma formation in backcross hybrids of seven genetic types, involving 1,110 fish and three pigment patterns. Highly significant associations of CDKN2X genotypes with such phenotypic effects suggests that this gene is a strong candidate for the classically defined DIFF tumor suppressor gene. Because published results have documented the involvement of the CDKN2A (p16, MTS1, and INK4A) tumor suppressor gene in human melanoma formation, the possibility of CDKN2 genes acting as tumor suppressors in both man and Xiphophorus is likely. Genes Chromosomes Cancer 22:210–220, 1998.

NONMAMMALIAN MODELS FOR SUNLIGHT CARCINOGENESIS : GENETIC ANALYSIS OF MELANOMA FORMATION IN XIPHOPHORUS HYBRID FISH

Abstract— Genetic hybrids of Xiphophorus fishes have been used for decades to study heritable melanoma formation. In these models, overexpression of pigmentation patterns from melanin‐producing pigment cells can lead to genetically regulated melanoma formation in backcross hybrids. In the best studied of these models, the Gordon‐Kosswig hybrid melanoma, tumors form spontaneously in all individuals of a subset of backcross hybrids between the platyflsh Xiphophorus maculatus Jp 163 A and the swordtail species Xiphophorus helleri. Backcross hybrids susceptible to melanoma formation inherit a sex‐linked oncogene, Xmrk, associated with the spotted dorsal (Sd) pigment pattern and have lost both copies of an autosomal gene, DIFF, from the X. maculatus parent. Spontaneous melanoma formation conforms to simple, two‐gene Mendelian inheritance in which DIFF behaves as a recessive tumor suppressor gene. Recently, Xiphophorus hybrids in which melanomas can be induced by UV and near‐UV visible light exposure have been described. We report here results of genetic linkage analysis of one of these Xiphophorus light‐inducible hybrid melanoma models, in backcross hybrids between the two platyflsh species X. maculatus Jp 163 B and Xiphophorus couchianus. Our linkage results provide the first estimate of recombination between the tumor suppressor locus, DIFF, and glycerate‐2‐dehydrogenase (GLYDH) in Xiphophorus linkage group V. Also, they demonstrate that DIFF regulates hyperplasia of spotted side (Sp) pigment cells in this hybrid model, analogous to its regulation of hyperplasia of Sd pigment cells in the “classical” Gordon‐Kosswig hybrid. Joint segregation analyses of melanoma‐bearing fish indicate that segregation of DIFF is genetically linked to melanoma induction by 405 nm light in this model but that induction of melanomas by UV wavelengths apparently does not depend on segregation of the DIFF locus.

DNA repair in the variable platyfish (Xiphophorus variatus) irradiated in vivo with ultraviolet B light.

Abstract— Dark‐ and light‐dependent DNA repair processes were studied in vivo in the variable platyfish, Xiphophorus vuriatus. Excision (dark) repair of the (6–4) photoproduct was more efficient than that of the cyclobutane dimer with ∼ 70% of the (6–4) photoproducts reniovcd by 24 h post‐UVB radiation compared to ∼30% of the cyclobutane dimers. Exposure to photoreactivating light resulted in rapid loss of most (>90%) of the cyclobutane dimers and increased excision repair of the (6–4) photoproduct. Preexposure to photoreactivating light 8 h prior to UVB radiation increased the rate of photoreactivation two‐fold.

Comparative structure and characterization of a CDKN2 gene in a Xiphophorus fish melanoma model

We have cloned, sequenced, and characterized the RNA expression properties of a fish CDKN2 gene from Xiphophorus helleri and X. maculatus. This gene, termed CDKN2X, shows a high degree of amino acid sequence similarity to members of the mammalian CDKN2 gene family, which includes the tumor suppressor loci CDKN2A (P16) and CDKN2B (P15). Comparative sequence analysis suggests that fish CDKN2X is similarly related to all four mammalian gene family members, and may represent a descendant of an ancestral prototypic CDKN2 gene. CDKN2X was mapped to a region on autosomal Xiphophorus linkage group V (LG V) known to contain the DIFF gene that acts as a tumor suppressor of melanoma formation in X. helleri/X. maculatus backcross hybrids. Thus, CDKN2X may be a candidate for the tumor suppressor DIFF gene. Here we have sequenced CDKN2X in both Xiphophorus species and have characterized its expression in normal and melanotic tissues within control and backcross hybrid fish. A simultaneous expressional analysis of the Xmrk-2 tyrosine kinase receptor gene, which is strongly implicated in melanomagenesis in this system, was also performed. RT – PCR analyses revealed that both genes were highly expressed in melanomas. For CDKN2X, this result contrasts numerous findings in human tumors including human melanoma in which either CDKN2A (P16) deactivation or LOH was observed.

Introgression among Apache, Cutthroat, and Rainbow Trout in Arizona

Abstract The Apache trout Oncorhynchus apache has become threatened through hybridization with introduced nonnative trouts, among other reasons. We used 10 isozyme locus polymorphisms, which were in the aggregate diagnostic for discrimination of alleles of Apache trout, rainbow trout Oncorhynchus mykiss, and cutthroat trout Oncorhynchus clarki, to assess extent and directionality of interspecific hybridization in 645 individuals from 31 wild populations within the historical range of Apache trout. Only 11 potentially unhybridized populations of Apache trout were found. Rainbow trout introgression was documented in 19 of the 31 populations, including at least two in which all individuals sampled were hybrids. In four of these introgressed populations, hybridization between cutthroat trout and Apache trout was detected; at two of these localities individuals with alleles from all three species were sampled. Apache-cutthroat hybrids were found at one locality where no rainbow trout alleles were sampled. In 1...

Multispecies hybridization among native and introduced centrarchid basses in central Texas.

Abstract Allele-frequency differences at 11 polymorphic protein loci provided discrete genetic markers with which to estimate the extent of hybridization among native and introduced centrarchid basses in central Texas streams. Native Guadalupe bass Micropterus treculi and northern largemouth bass M. salmoides salmoides now coexist with introduced Florida largemouth bass M. s. floridanus and smallmouth bass M. dolomieui in these streams. Interspecific hybridization was detected in three of four populations from the Blanco and San Marcos rivers, hybrids making up at least 31.8% of the individuals sampled. Complex hybridization patterns were evident, because F1, F2, and backcross hybrids were detected. At least one individual exhibited genetic markers of largemouth, smallmouth, and Guadalupe bass. Extensive multispecies hybridization threatens the survival of the endemic Guadalupe bass.

Genetic analysis of susceptibility to spontaneous and UV-induced carcinogenesis in Xiphophorus hybrid fish.

Abstract:Xiphophorus interspecies hybrids provide genetically controlled models of tumor formation. Spontaneous melanomas form in first-generation backcross (BC1) hybrids produced from backcrossing F1 hybrids derived from the platyfish X. maculatus Jp 163 A and the swordtail X. helleri to the X. helleri parental strain (the Gordon-Kosswig hybrid cross). Nodular melanomas originate in the dorsal fin from cells constituting the spotted dorsal (Sd) pigment pattern. A parallel genetic cross, with X. maculatus Jp 163 B, exhibits the spotted side (Sp) pigment pattern instead of Sd, and produces BC1 hybrids exhibiting a much lower frequency of spontaneous melanoma formation. These hybrids are susceptible to melanoma development if irradiated with UV light as fry. Other hybrids involving these two strains of X. maculatus and different swordtail and platyfish backcross parents also have been investigated as potential tumor models, and show differing susceptibilities to UV-induced and spontaneous melanomas. Genotyping of individual BC1 hybrids from several Xiphophorus crosses has implicated a locus, CDKN2X (a Xiphophorus homologue of the mammalian CDKN2 gene family, residing on Xiphophorus linkage group V), in enhancing pigmentation and the susceptibility to spontaneous and UV-induced melanoma formation in BC1 hybrids from some crosses, but not others. Homozygosity for X. helleri and X. couchianusCDKN2X alleles in BC1 hybrids can predispose individuals to melanoma, but this susceptibility is modified in other crosses depending both on the contributing sex-linked pigment pattern locus from X. maculatus (Sd or Sp), and the genetic constitution of the backcross parent. Xiphophorus BC1 hybrids constitute unique genetic models offering the potential to analyze the contributions of specific genes to spontaneous and induced tumor formation in different, but comparable genetic backgrounds.

Genetic Analysis of Neoplasia Induced by N-Nitroso-N-methylurea in Xiphophorus Hybrid Fish

Abstract: Interspecific crosses within the genus Xiphophorus have historically been used to study the genetic aspects of melanoma formation. Melanomas typically occur as a result of deregulation of polymorphic, naturally occurring macromelanophore pigment patterns. Hybrid crosses also have been used to study the inducibility of melanoma by physical sources (such as UV light) and chemicals (such as N-methyl-N-nitrosourea, MNU). We previously defined a genomic region that is implicated in fish melanomagenesis and identified a candidate tumor suppressor gene (CDKN2X) within this genomic area. Highly significant associations between BC1-hybrid CDKN2X genotypes and UV-induced melanoma formation exist in a backcross produced from 2 inbred parental lines. However, when BC1 hybrids are exposed to MNU as the tumor induction agent, a significant association between inheritance of CDKN2X alleles and tumor development is not observed. These data suggest there is mechanistic and genetic heterogeneity in melanomas derived from different etiologies within BC1 hybrid fish.