The Linkage Map of Xiphophorus Fishes

Xiphophorus and Medaka Cancer Models

Besides recently developed zebrafish cancer models, other fish species have been employed for many years as cancer models in laboratory studies. Two models, namely in Xiphophorus and medaka have proven useful in providing important clues to cancer etiology. Medaka is a complementary model to zebrafish in many areas of research since it offers similar resources and experimental tools. Xiphophorus provides the advantages of a natural ("evolutionary mutant") model with established genetics. Xiphophorus hybrids can develop spontaneous and radiation or carcinogen induced cancers. This chapter describes the tumor models in both species, which mainly focus on melanoma, and summarizes the main findings and future research directions.

The genetic map of Xiphophorus fishes represented by 24 multipoint linkage groups

Hybrids between distinct Xiphophorus species have been utilized for over 70 years to study melanoma and other neoplasms that can develop spontaneously in hybrid offspring. Genetic linkage mapping has proven to be important in delineating genomic areas that harbor oncogenes and tumor suppressors. Within this report, two parallel backcrosses have been utilized to generate a genetic linkage map for Xiphophorus fishes. Isozyme/allozyme, RFLP and PCR-based mapping techniques, including AP-PCR/RAPDs and microsatellite loci were utilized. The derived linkage map provides a total of 403 mapped polymorphisms distributed among 24 linkage groups, representative of 24 acro- and telocentric chromosome pairs. Genomic coverage is approximately one marker per 5.8 cM. Detailed genotypic analysis of the utilized hybrids revealed two areas of the genome that show significant segregation distortion. Loci within the linkage group harboring the sex determining locus (LG 24) and an autosomal linkage group (LG 21) show highly significant deviations from Mendelian expectations. This phenomenon is not present in a hybrid cross that utilizes a different backcross hybrid progenitor species. The derived map with sequence-tagged markers provides a framework for physical map generation, large-scale genomic sequencing and will further enable cross-genome comparisons of vertebrate genomes.

A microsatellite genetic linkage map for Xiphophorus

Interspecies hybrids between distinct species of the genus Xiphophorus are often used in varied research investigations to identify genomic regions associated with the inheritance of complex traits. There are 24 described Xiphophorus species and a greater number of pedigreed strains; thus, the number of potential interspecies hybrid cross combinations is quite large. Previously, select Xiphophorus experimental crosses have been shown to exhibit differing characteristics between parental species and among the hybrid fishes derived from crossing them, such as widely differing susceptibilities to chemical or physical agents. For instance, genomic regions harboring tumor suppressor and oncogenes have been identified via linkage association of these loci with a small set of established genetic markers. The power of this experimental strategy is related to the number of genetic markers available in the Xiphophorus interspecies cross of interest. Thus, we have undertaken the task of expanding the suite of easily scored markers by characterization of Xiphophorus microsatellite sequences. Using a cross between Xiphophorus maculatus and X. andersi, we report a linkage map predominantly composed of microsatellite markers. All 24 acrocentric chromosome sets of Xiphophorus are represented in the assembled linkage map with an average intergenomic distance of 7.5 cM. Since both male and female F1 hybrids were used to produce backcross progeny, these recombination rates were compared between "male" and "female" maps. Although several genomic regions exhibit differences in map length, male- and female-derived maps are similar. Thus Xiphophorus, in contrast to zebrafish, Danio rerio, and several other vertebrate species, does not show sex-specific differences in recombination. The microsatellite markers we report can be easily adapted to any Xiphophorus interspecies and some intraspecies crosses, and thus provide a means to directly compare results derived from independent experiments.

Structural organization, mapping, characterization and evolutionary relationships of CDKN2 gene family members in Xiphophorus fishes

Xiphophorus fishes and their hybrids are used as models for the study of melanoma and other diseases. The cyclin-dependent kinase inhibitor gene family in humans is comprised of four members, including CDKN2A (P16), and dysregulation of this gene is implicated in numerous neoplasms including melanomas. We have investigated the status of the gene family in the southern platyfish X. maculatus. Xiphophorus harbors at least two such loci, which we now term CDKN2A/B and CDKN2D. Both loci map to Xiphophorus linkage group 5, a genomic area that has long been known to harbor the DIFF tumor suppressor locus. Within this report, we report on the complete cloning, genomic exon/intron boundary delineation, linkage mapping and expressional characteristics of Xiphophorus CDKN2D. We also compare and contrast this expression to that of the previously isolated CDKN2AB locus in normal and neoplastic tissues derived from non-hybrid and hybrid fishes. The hypothetical evolutionary relationships of gene family members and their involvement in melanoma is evaluated. In comparison to CDKN2A/B, the RNA expression of Xiphophorus CDKN2D differs in normal tissues and is not associated with melanotic/pathologic tissues, confirming functional divergence between obvious homologues.

Cloning and analysis of a FoxO transcription factor from Xiphophorus

Melanoma development in the fish Xiphophorus is determined, at least in part, by overexpression and activation of the Xmrk-2 oncogene, which triggers a variety of signal transduction pathways resulting in altered cell cycle control. We have begun analysing transcription factors which may link Xmrk-2 with regulation of cell proliferation or apoptosis. Towards this end, we have cloned an FKHR (FoxO sub-family) homolog from Xiphophorus maculatus. The isolated clone is a 2.7 kb cDNA encoding a predicted protein of 664 amino acids. The gene, which we have named FoxO5, maps to Xiphophorus Linkage Group XV. The protein product can be categorized within a branch of the FOXO sub-class, which includes: Danio rerio zFKHR (foxo5), Homo sapiens FKHR-L1 (FoxO3a) and Mus musculus FKHR2 (Foxo3). Notably, the Forkhead DNA binding domain, three Akt consensus phosphorylation sites and a carboxy-terminal minimal activation domain are each highly conserved. A mutated FoxO5 protein with disrupted Akt phosphorylation sites inhibits proliferation, but the wild-type protein fails to do so, when exogenously expressed in Xiphophorus cells derived from a melanoma. The same mutated protein predominantly localizes to the nucleus, yet the wild-type protein seldom does. Further characterization of Xiphophorus FoxO5 will contribute to understanding the molecular basis of carcinogenesis in these species.

Parental 5-methylcytosine methylation patterns are stable upon inter-species hybridization of Xiphophorus (Teleostei: Poeciliidae) fish

Cytosine methylation appears to be established as an important DNA base modification involved in regulation of gene expression but is poorly understood from an evolutionary viewpoint. Xiphophorus progeny from inter-species crosses and backcrosses that are utilized in contemporary tumor induction studies were analyzed for cytosine methylation pattern inheritance using Southern blot analyses. Methylation patterns at CCGG sequences of 411 independent chromosomes in three distinct inter-species crosses were analyzed. In every case the non-recurrent parental methylation pattern remained unaltered for each of the genes studied, once introduced into the recurrent parental genetic background. Through F(1) inter-species hybridization and succeeding meiosises leading to first generation (BC(1)) and second generation (BC(2)) backcross hybrid progeny, we demonstrate that parental species methylation patterns are stable.

Xiphophorus interspecies hybrids as genetic models of induced neoplasia

Fishes of the genus Xiphophorus (platyfishes and swordtails) are small, internally fertilizing, livebearing, and derived from freshwater habitats in Mexico, Guatemala, Belize, and Honduras. Scientists have used these fishes in cancer research studies for more than 70 yr. The genus is presently composed of 22 species that are quite divergent in their external morphology. Most cancer studies using Xiphophorus use hybrids, which can be easily produced by artificial insemination. Phenotypic traits, such as macromelanophore pigment patterns, are often drastically altered as a result of lack of gene regulation within hybrid fishes. These fish can develop large exophytic melanomas as a result of upregulated expression of these pigment patterns. Because backcross hybrid fish are susceptible to the development of melanoma and other neoplasms, they can be subjected to potentially deleterious chemical and physical agents. It is thus possible to use gene mapping and cloning methodologies to identify and characterize oncogenes and tumor suppressors implicated in spontaneous or induced neoplasia. This article reviews the history of cancer research using Xiphophorus and recent developments regarding DNA repair capabilities, mapping, and cloning of candidate genes involved in neoplastic phenotypes. The particular genetic complexity of melanoma in these fishes is analyzed and reviewed.

Background and Overview of Comparative Genomics

Comparative genomics---the cross-referencing of information on genome organization between species---provides an additional dimension to the Human Genome Project and can derive much information from it for the benefit of animal health and animal breeding. Arrangements of genes and other DNA sequences may be determined by a variety of genetic and physical techniques, at resolutions from the gross cytological level to the level of the single base pair. Gross arrangements and rearrangements can also be charted by comparative chromosome painting. Genome organization may then be compared across mammal---and other vertebrate---species. Genetic mapping is well advanced in several livestock species as well as rodent model species, and outline maps are available for at least 30 mammal species in eight orders. At the time of this writing, maps are being rapidly constructed for chicken and fish species. Comparisons, even over vast evolutionary time scales, show that the mammal genome---indeed, the vertebrate genome---has been highly conserved. Thus, information about location and function of genes is directly transferable across species and should greatly accelerate the search for genes that specify inherited diseases in domestic mammals and humans as well as genes that specify economically important traits.

Melanoma loss-of-function mutants in Xiphophorus caused by Xmrk-oncogene deletion and gene disruption by a transposable element

The overexpression of the Xmrk oncogene (ONC-Xmrk) in pigment cells of certain Xiphophorus hybrids has been found to be the primary change that results in the formation of malignant melanoma. Spontaneous mutant stocks have been isolated that have lost the ability to induce tumor formation when crossed with Xiphophorus helleri. Two of these loss-of-function mutants were analyzed for genetic defects in ONC-Xmrk's. In the lof-1 mutant a novel transposable element, TX-1, has jumped into ONC-Xmrk, leading to a disruption of the gene and a truncated protein product lacking the carboxyterminal domain of the receptor tyrosine kinase. TX-1 is obviously an active LTR-containing retrotransposon in Xiphophorus that was not found in other fish species outside the family Poeciliidae. Surprisingly, it does not encode any protein, suggesting the existence of a helper function for this retroelement. In the lof-2 mutant the entire ONC-Xmrk gene was found to be deleted. These data show that ONC-Xmrk is indeed the tumor-inducing gene of Xiphophorus and thus the critical constituent of the tumor (Tu) locus.

Identification and mapping of two divergent, unlinked major histocompatibility complex class II B genes in Xiphophorus fishes

We have isolated two major histocompatibility complex (MHC) class II B genes from the inbred fish strain Xiphophorus maculatus Jp 163 A. We mapped one of these genes, designated here as DXB, to linkage group III, linked to a malic enzyme locus, also syntenic with human and mouse MHC. Comparison of genomic and cDNA clones shows the gene consists of six exons and five introns. The encoded beta1 domain has three amino acids deleted and a cytoplasmic tail nine amino acids longer than in other teleost class II beta chains, more similar to HLA-DRB, clawed frog Xela-F3, and nurse shark Gici-B. Key residues for disulfide bonds, glycosylation, and interaction with alpha chains are conserved. These same features are also present in a swordtail (Xiphophorus helleri) genomic DXB PCR clone. A second type of class II B clone was amplified by PCR from X. maculatus and found to be orthologous to class II genes identified in other fishes. This DAB-like gene is 63% identical to the X. maculatus DXB sequence in the conserved beta2-encoding exon and was mapped to new unassigned linkage group LG U24. The DXB gene, then, represents an unlinked duplicated locus not previously identified in teleosts.