Genetic parameters and quantitative trait loci analysis associated with body size and timing at metamorphosis into glass eels in captive-bred Japanese eels (Anguilla japonica).

The Japanese eel (Anguilla japonica) is among the most important aquaculture fish species in Eastern Asia. The present study aimed to identify the genetic parameters underlying body size and the timing at metamorphosis from leptocephali to glass eels in captive-bred Japanese eels, with the intent to foster sustainable development. Larvae from a partly factorial cross (14 sires × 11 dams) were reared until the point of metamorphosis into glass eels. In these organisms, we observed moderate heritability and mild genetic correlations among traits related to body size (h2 = 0.16-0.33) and timing at metamorphosis (h2 = 0.36-0.41). In an F1 full-sib family, quantitative trait loci (QTL) mapping for these traits identified one significant (genome-wide P < 0.05) and five suggestive QTLs (chromosome-wide P < 0.05). These results suggest that in the Japanese eel, metamorphic traits exhibit a polygenic genetic structure comprising many QTLs with small effects. In addition, we updated the genetic linkage map for the Japanese eel and integrated it with our newly constructed de novo genome assembly. The information and tools generated from this study will contribute to the development of freshwater eel genetics and genomics.

Identification of Sex-associated SNPs of Greater Amberjack (Seriola dumerili).

The sex determination systems of fish are highly diverse compared with those of mammals. Thus, performing investigations using nonmodel fish species helps to understand the highly diverse sex determination systems of fish. Because greater amberjack (Seriola dumerili) is one of the most important edible fish globally and knowledge of its sex determination system is economically important in the field of aquaculture, we are interested in the mechanisms of sex determination of Seriola species. In this study, we identified sex-associated SNPs of greater amberjack using SNP information of 10 males and 10 females by an association test. We determined that the sex-associated SNPs were on chromosome 12 and mainly covered with two scaffolds (about 7.1 Mbp). Genotypes of sex-associated SNPs indicated that females are the heterogametic sex (ZZ/ZW). Furthermore, we compared the genomic structure of greater amberjack with those of Japanese amberjack (Seriola quinqueradiata), California yellowtail (Seriola dorsalis), and medaka (Oryzias latipes). Whole-genome alignments and synteny analysis indicated that the sex determination system of greater amberjack is markedly different from that of medaka and implied that the sex determination system is conserved in the Seriola species.

Whole Genome Sequencing of Greater Amberjack (Seriola dumerili) for SNP Identification on Aligned Scaffolds and Genome Structural Variation Analysis Using Parallel Resequencing.

Greater amberjack (Seriola dumerili) is distributed in tropical and temperate waters worldwide and is an important aquaculture fish. We carried out de novo sequencing of the greater amberjack genome to construct a reference genome sequence to identify single nucleotide polymorphisms (SNPs) for breeding amberjack by marker-assisted or gene-assisted selection as well as to identify functional genes for biological traits. We obtained 200 times coverage and constructed a high-quality genome assembly using next generation sequencing technology. The assembled sequences were aligned onto a yellowtail (Seriola quinqueradiata) radiation hybrid (RH) physical map by sequence homology. A total of 215 of the longest amberjack sequences, with a total length of 622.8 Mbp (92% of the total length of the genome scaffolds), were lined up on the yellowtail RH map. We resequenced the whole genomes of 20 greater amberjacks and mapped the resulting sequences onto the reference genome sequence. About 186,000 nonredundant SNPs were successfully ordered on the reference genome. Further, we found differences in the genome structural variations between two greater amberjack populations using BreakDancer. We also analyzed the greater amberjack transcriptome and mapped the annotated sequences onto the reference genome sequence.

Identification of Quantitative Trait Loci for Resistance to RSIVD in Red Sea Bream (Pagrus major).

Red sea bream iridoviral disease (RSIVD) is a major viral disease in red sea bream farming in Japan. Previously, we identified one candidate male individual of red sea bream that was significantly associated with convalescent individuals after RSIVD. The purpose of this study is to identify the quantitative trait loci (QTL) linked to the RSIVD-resistant trait for future marker-assisted selection (MAS). Two test families were developed using the candidate male in 2014 (Fam-2014) and 2015 (Fam-2015). These test families were challenged with RSIV, and phenotypes were evaluated. Then, de novo genome sequences of red sea bream were obtained through next-generation sequencing, and microsatellite markers were searched and selected for linkage map construction. One immune-related gene, MHC class IIß, was also used for linkage map construction. Of the microsatellite markers searched, 148 and 197 were mapped on 23 and 27 linkage groups in the female and male linkage maps, respectively, covering approximately 65% of genomes in both sexes. One QTL linked to an RSIVD-resistant trait was found in linkage group 2 of the candidate male in Fam-2014, and the phenotypic variance of the QTL was 31.1%. The QTL was closely linked to MHC class IIß. Moreover, the QTL observed in Fam-2014 was also significantly linked to an RSIVD-resistant trait in the candidate male of Fam-2015. Our results suggest that the RSIVD-resistant trait in the candidate male was controlled by one major QTL closely linked to the MHC class IIß gene and could be useful for MAS of red sea bream.

A novel C-type lectin gene is a strong candidate gene for Benedenia disease resistance in Japanese yellowtail, Seriola quinqueradiata.

Little is known about mechanisms of resistance to parasitic diseases in marine finfish. Benedenia disease is caused by infection by the monogenean parasite Benedenia seriolae. Previous quantitative trait locus (QTL) analyses have identified a major QTL associated with resistance to Benedenia disease in linkage group Squ2 of the Japanese yellowtail/amberjack Seriola quinqueradiata. To uncover the bioregulatory mechanism of Benedenia disease resistance, complete Illumina sequencing of BAC clones carrying genomic DNA for the QTL region in linkage group Squ2 was performed to reveal a novel C-type lectin in this region. Expression of the mRNA of this C-type lectin was detected in skin tissue parasitized by B. seriolae. Scanning for single nucleotide polymorphisms (SNPs) uncovered a SNP in the C-type lectin/C-type lectin-like domain that was significantly associated with B. seriolae infection levels. These results strongly suggest that the novel C-type lectin gene controls resistance to Benedenia disease in Japanese yellowtails.

Detection of Growth-Related Quantitative Trait Loci and High-Resolution Genetic Linkage Maps Using Simple Sequence Repeat Markers in the Kelp Grouper (Epinephelus bruneus).

To initiate breeding programs for kelp grouper (Epinephelus bruneus), the establishment of genetic linkage maps becomes essential accompanied by the search for quantitative trait loci (QTLs) that may be utilized in selection programs. We constructed a high-resolution genetic linkage map using 1055 simple sequence repeat (SSR) markers in an F1 family. Genome-wide and chromosome-wide significances of growth-related QTLs (body weight: BW and total length: TL) were detected using non-parametric mapping, Kruskal-Wallis analysis, simple interval mapping (IM), and a permutation test (PT). Two stages and two families of fish were used to confirm the QTL regions. Ultimately, 714 SSR markers were matched that evenly covered the 24 linkage groups. In total, 509 and 512 markers were localized to the female and male maps, respectively. The genome lengths were approximately 1475.95 and 1370.39 cM and covered 84.68 and 83.21 % of the genome, with an average interval of 4.1 and 4.0 cM, in females and males, respectively. One major QTL affecting BW and TL was found on linkage group EBR 17 F that identified for 1 % of the genome-wide significance and accounted for 14.6-18.9 % and 14.7-18.5 % of the phenotypic variance, and several putative QTL with 5 % chromosome-wide significance were detected on eight linkage groups. Furthermore, the confirmed results of the regions harboring the major and putative QTLs showed consistent significant experiment-wide values of 1 and 5 % as well as a chromosome-wide value of 5 %. We identified growth-related QTLs that could be applied to find candidate genes for growth traits in further studies and potentially useful in marker assisted selection (MAS) breeding.

Detection of Growth-Related Quantitative Trait Loci and High-Resolution Genetic Linkage Maps Using Simple Sequence Repeat Markers in the Kelp Grouper (Epinephelus bruneus).

To initiate breeding programs for kelp grouper (Epinephelus bruneus), the establishment of genetic linkage maps becomes essential accompanied by the search for quantitative trait loci that may be utilized in selection programs. We constructed a high-resolution genetic linkage map using 1055 simple sequence repeat (SSR) markers in an F1 family. Genome-wide and chromosome-wide significances of growth-related quantitative trait loci (QTLs) (body weight (BW) and total length (TL)) were detected using non-parametric mapping, Kruskal-Wallis (K-W) analysis, simple interval mapping (IM) and a permutation test (PT). Two stages and two families of fish were used to confirm the QTL regions. Ultimately, 714 SSR markers were matched that evenly covered the 24 linkage groups. In total, 509 and 512 markers were localized to the female and male maps, respectively. The genome lengths were approximately 1475.95 and 1370.39 cM and covered 84.68 and 83.21% of the genome, with an average interval of 4.1 and 4.0 cM, in females and males, respectively. One major QTL affecting BW and TL was found on linkage group EBR 17F that identified for 1% of the genome-wide significance and accounted for 14.6-18.9 and 14.7-18.5% of the phenotypic variance, and several putative QTL with 5% chromosome-wide significance were detected on eight linkage groups. Furthermore, the confirmed results of the regions harboring the major and putative QTLs showed consistent significant experiment-wide values of 1 and 5% as well as a chromosome-wide value of 5%. We identified growth-related QTLs that could be applied to find candidate genes for growth traits in further studies, and potentially useful in MAS breeding.

Identification of Sex-Linked SNPs and Sex-Determining Regions in the Yellowtail Genome.

Unlike the conservation of sex-determining (SD) modes seen in most mammals and birds, teleost fishes exhibit a wide variety of SD systems and genes. Hence, the study of SD genes and sex chromosome turnover in fish is one of the most interesting topics in evolutionary biology. To increase resolution of the SD gene evolutionary trajectory in fish, identification of the SD gene in more fish species is necessary. In this study, we focused on the yellowtail, a species widely cultivated in Japan. It is a member of family Carangidae in which no heteromorphic sex chromosome has been observed, and no SD gene has been identified to date. By performing linkage analysis and BAC walking, we identified a genomic region and SNPs with complete linkage to yellowtail sex. Comparative genome analysis revealed the yellowtail SD region ancestral chromosome structure as medaka-fugu. Two inversions occurred in the yellowtail linage after it diverged from the yellowtail-medaka ancestor. An association study using wild yellowtails and the SNPs developed from BAC ends identified two SNPs that can reasonably distinguish the sexes. Therefore, these will be useful genetic markers for yellowtail breeding. Based on a comparative study, it was suggested that a PDZ domain containing the GIPC protein might be involved in yellowtail sex determination. The homomorphic sex chromosomes widely observed in the Carangidae suggest that this family could be a suitable marine fish model to investigate the early stages of sex chromosome evolution, for which our results provide a good starting point.

Second generation physical and linkage maps of yellowtail (Seriola quinqueradiata) and comparison of synteny with four model fish.

BACKGROUND: Physical and linkage maps are important aids for the assembly of genome sequences, comparative analyses of synteny, and to search for candidate genes by quantitative trait locus analysis. Yellowtail, Seriola quinqueradiata, is an economically important species in Japanese aquaculture, and genetic information will be useful for DNA-assisted breeding. We report the construction of a second generation radiation hybrid map, its synteny analysis, and a second generation linkage map containing SNPs (single nucleotide polymorphisms) in yellowtail. RESULTS: Approximately 1.4 million reads were obtained from transcriptome sequence analysis derived from 11 tissues of one individual. To identify SNPs, cDNA libraries were generated from a pool of 500 whole juveniles, and the gills and kidneys of 100 adults. 9,356 putative SNPs were detected in 6,025 contigs, with a minor allele frequency ≥ 25%. The linkage and radiation hybrid maps were constructed based on these contig sequences. 2,081 markers, including 601 SNPs markers, were mapped onto the linkage map, and 1,532 markers were mapped in the radiation hybrid map. CONCLUSIONS: The second generation linkage and physical maps were constructed using 6,025 contigs having SNP markers. These maps will aid the de novo assembly of sequencing reads, linkage studies and the identification of candidate genes related to important traits. The comparison of marker contigs in the radiation hybrid map indicated that yellowtail is evolutionarily closer to medaka than to green-spotted pufferfish, three-spined stickleback or zebrafish. The synteny analysis may aid studies of chromosomal evolution in yellowtail compared with model fish.

Construction of a high-coverage bacterial artificial chromosome library and comprehensive genetic linkage map of yellowtail Seriola quinqueradiata.

BACKGROUND: Japanese amberjack/yellowtail (Seriola quinqueradiata) is a commonly cultured marine fish in Japan. For cost effective fish production, a breeding program that increases commercially important traits is one of the major solutions. In selective breeding, information of genetic markers is useful and sufficient to identify individuals carrying advantageous traits but if the aim is to determine the genetic basis of the trait, large insert genomic DNA libraries are essential. In this study, toward prospective understanding of genetic basis of several economically important traits, we constructed a high-coverage bacterial artificial chromosome (BAC) library, obtained sequences from the BAC-end, and constructed comprehensive female and male linkage maps of yellowtail using Simple Sequence Repeat (SSR) markers developed from the BAC-end sequences and a yellowtail genomic library. RESULTS: The total insert length of the BAC library we constructed here was estimated to be approximately 11 Gb and hence 16-times larger than the yellowtail genome. Sequencing of the BAC-ends showed a low fraction of repetitive sequences comparable to that in Tetraodon and fugu. A total of 837 SSR markers developed here were distributed among 24 linkage groups spanning 1,026.70 and 1,057.83 cM with an average interval of 4.96 and 4.32 cM in female and male map respectively without any segregation distortion. Oxford grids suggested conserved synteny between yellowtail and stickleback. CONCLUSIONS: In addition to characteristics of yellowtail genome such as low repetitive sequences and conserved synteny with stickleback, our genomic and genetic resources constructed and revealed here will be powerful tools for the yellowtail breeding program and also for studies regarding the genetic basis of traits.

A ddRAD-based genetic map and its integration with the genome assembly of Japanese eel (Anguilla japonica) provides insights into genome evolution after the teleost-specific genome duplication.

BACKGROUND: Recent advancements in next-generation sequencing technology have enabled cost-effective sequencing of whole or partial genomes, permitting the discovery and characterization of molecular polymorphisms. Double-digest restriction-site associated DNA sequencing (ddRAD-seq) is a powerful and inexpensive approach to developing numerous single nucleotide polymorphism (SNP) markers and constructing a high-density genetic map. To enrich genomic resources for Japanese eel (Anguilla japonica), we constructed a ddRAD-based genetic map using an Ion Torrent Personal Genome Machine and anchored scaffolds of the current genome assembly to 19 linkage groups of the Japanese eel. Furthermore, we compared the Japanese eel genome with genomes of model fishes to infer the history of genome evolution after the teleost-specific genome duplication. RESULTS: We generated the ddRAD-based linkage map of the Japanese eel, where the maps for female and male spanned 1748.8 cM and 1294.5 cM, respectively, and were arranged into 19 linkage groups. A total of 2,672 SNP markers and 115 Simple Sequence Repeat markers provide anchor points to 1,252 scaffolds covering 151 Mb (13%) of the current genome assembly of the Japanese eel. Comparisons among the Japanese eel, medaka, zebrafish and spotted gar genomes showed highly conserved synteny among teleosts and revealed part of the eight major chromosomal rearrangement events that occurred soon after the teleost-specific genome duplication. CONCLUSIONS: The ddRAD-seq approach combined with the Ion Torrent Personal Genome Machine sequencing allowed us to conduct efficient and flexible SNP genotyping. The integration of the genetic map and the assembled sequence provides a valuable resource for fine mapping and positional cloning of quantitative trait loci associated with economically important traits and for investigating comparative genomics of the Japanese eel.

Construction of a radiation hybrid panel and the first yellowtail (Seriola quinqueradiata) radiation hybrid map using a nanofluidic dynamic array.

BACKGROUND: Yellowtail (Seriola quinqueradiata) are an economically important species in Japan. However, there are currently no methods for captive breeding and early rearing for yellowtail. Thus, the commercial cultivation of this species is reliant upon the capture of wild immature fish. Given this, there is a need to develop captive breeding techniques to reduce pressure on wild stocks and facilitate the sustainable development of yellowtail aquaculture. We constructed a whole genome radiation hybrid (RH) panel for yellowtail gene mapping and developed a framework physical map using a nanofluidic dynamic array to use SNPs (single nucleotide polymorphisms) in ESTs (expressed sequence tags) for the DNA-assisted breeding of yellowtail. RESULTS: Clonal RH cell lines were obtained after ionizing radiation; specifically, 78, 64, 129, 55, 42, and 53 clones were isolated after treatment with 3,000, 4,000, 5,000, 6,000, 8,000, or 10,000 rads, respectively. A total of 421 hybrid cell lines were obtained by fusion with mouse B78 cells. Ninety-four microsatellite markers used in the genetic linkage map were genotyped using the 421 hybrid cell lines. Based upon marker retention and genome coverage, we selected 93 hybrid cell lines to form an RH panel. Importantly, we performed the first genotyping of yellowtail markers in an RH panel using a nanofluidic dynamic array (Fluidigm, CA, USA). Then, 580 markers containing ESTs and SNPs were mapped in the first yellowtail RH map. CONCLUSIONS: We successfully developed a yellowtail RH panel to facilitate the localization of markers. Using this, a framework RH map was constructed with 580 markers. This high-density physical map will serve as a useful tool for the identification of genes related to important breeding traits using genetic structural information, such as conserved synteny. Moreover, in a comparison of 30 sequences in the RH group 1 (SQ1), yellowtail appeared to be evolutionarily closer to medaka and the green-spotted pufferfish than to zebrafish. We suggest that synteny analysis may be potentially useful as a tool to investigate chromosomal evolution by comparison with model fish.

Growth of cyprinid herpesvirus 2 (CyHV-2) in cell culture and experimental infection of goldfish Carassius auratus.

Herpesviral haematopoietic necrosis has caused great economic damage to goldfish Carassius auratus aquaculture in Japan. The existence of cyprinid herpesvirus 2 (CyHV-2), the causative agent, has also been reported from several other countries. To prevent spread to other areas, basic virological information such as viral kinetics in infected fish is essential. Experimental infection trials using reliably prepared CyHV-2 for defining viral kinetics are difficult to carry out because successful and sustainable propagation of this virus in cell culture has previously been limited. Here we describe a method for sustainable propagation of CyHV-2 in cell culture, and the results of fish infection experiments using the propagated virus. We found that goldfish fin (GFF) cells and standard Ryukin Takafumi (SRTF) cells established from goldfish fin can be used for continuous propagation of CyHV-2. Experimental infections using 2 varieties of goldfish, Ryukin and Edonishiki, were performed with the virus passaged 7 times in GFF cells. In transmission experiments with water temperature at 20°C, cumulative mortality was 30% in Ryukin infected by immersion, and 90 and 100% in Edonishiki and Ryukin intraperitoneally injected with the virus, respectively. In an experiment carried out at 25°C, 90% of Edonishiki challenged by immersion died. PCR detection of viral DNA from the organs of infected fish showed that systemic infection occurs and also that the kidney is a main viral multiplication site. Moreover, CyHV-2 was successfully re-isolated in GFF cells from the dead fish.

Quantitative trait loci (QTL) associated with resistance to a monogenean parasite (Benedenia seriolae) in yellowtail (Seriola quinqueradiata) through genome wide analysis.

Benedenia infections caused by the monogenean fluke ectoparasite Benedenia seriolae seriously impact marine finfish aquaculture. Genetic variation has been inferred to play a significant role in determining the susceptibility to this parasitic disease. To evaluate the genetic basis of Benedenia disease resistance in yellowtail (Seriola quinqueradiata), a genome-wide and chromosome-wide linkage analyses were initiated using F1 yellowtail families (n = 90 per family) based on a high-density linkage map with 860 microsatellite and 142 single nucleotide polymorphism (SNP) markers. Two major quantitative trait loci (QTL) regions on linkage groups Squ2 (BDR-1) and Squ20 (BDR-2) were identified. These QTL regions explained 32.9-35.5% of the phenotypic variance. On the other hand, we investigated the relationship between QTL for susceptibility to B. seriolae and QTL for fish body size. The QTL related to growth was found on another linkage group (Squ7). As a result, this is the first genetic evidence that contributes to detailing phenotypic resistance to Benedenia disease, and the results will help resolve the mechanism of resistance to this important parasitic infection of yellowtail.

A second generation genetic linkage map of Japanese flounder (Paralichthys olivaceus).

BACKGROUND: Japanese flounder (Paralichthys olivaceus) is one of the most economically important marine species in Northeast Asia. Information on genetic markers associated with quantitative trait loci (QTL) can be used in breeding programs to identify and select individuals carrying desired traits. Commercial production of Japanese flounder could be increased by developing disease-resistant fish and improving commercially important traits. Previous maps have been constructed with AFLP markers and a limited number of microsatellite markers. In this study, improved genetic linkage maps are presented. In contrast with previous studies, these maps were built mainly with a large number of codominant markers so they can potentially be used to analyze different families and populations. RESULTS: Sex-specific genetic linkage maps were constructed for the Japanese flounder including a total of 1,375 markers [1,268 microsatellites, 105 single nucleotide polymorphisms (SNPs) and two genes]; 1,167 markers are linked to the male map and 1,067 markers are linked to the female map. The lengths of the male and female maps are 1,147.7 cM and 833.8 cM, respectively. Based on estimations of map lengths, the female and male maps covered 79 and 82% of the genome, respectively. Recombination ratio in the new maps revealed F:M of 1:0.7. All linkage groups in the maps presented large differences in the location of sex-specific recombination hot-spots. CONCLUSIONS: The improved genetic linkage maps are very useful for QTL analyses and marker-assisted selection (MAS) breeding programs for economically important traits in Japanese flounder. In addition, SNP flanking sequences were blasted against Tetraodon nigroviridis (puffer fish) and Danio rerio (zebrafish), and synteny analysis has been carried out. The ability to detect synteny among species or genera based on homology analysis of SNP flanking sequences may provide opportunities to complement initial QTL experiments with candidate gene approaches from homologous chromosomal locations identified in related model organisms.

Utilization of mammalian cells for efficient and reliable evaluation of specificity of antibodies to unravel the cellular function of mKIAA proteins.

Complementary DNA (cDNA) clones for human KIAA genes have been isolated as long cDNAs (>4 kb) with unknown functions. To facilitate the functional analysis of these human clones, we have isolated and determined the structures of their respective mouse homologues (mKIAA genes). Furthermore, we have comprehensively raised antibodies against the translated mKIAA proteins in order to establish a platform for their functional analysis. Since the specificity of these antibodies is critical for subsequent analyses of protein function, here we introduce two assays utilizing mammalian cells to improve their evaluation. First, we have established a semi-high-throughput production of C-terminally FLAG epitope-tagged proteins for Western blotting using specially designed mammalian expression vectors. Secondly, we have utilized immunofluorescence staining of mouse cells to analyze the subcellular localization of endogenous mKIAA proteins. Importantly, these methods allow us to detect potential posttranslational modification of the mKIAA/KIAA proteins and to predict their biological function based on their subcellular localization.

A comparative analysis of the rainbow trout genome with 2 other species of fish (Arctic charr and Atlantic salmon) within the tetraploid derivative Salmonidae family (subfamily: Salmoninae).

We updated the genetic map of rainbow trout (Oncorhynchus mykiss) for 2 outcrossed mapping panels, and used this map to assess the putative chromosome structure and recombination rate differences among linkage groups. We then used the rainbow trout sex-specific maps to make comparisons with 2 other ancestrally polyploid species of salmonid fishes, Arctic charr (Salvelinus alpinus) and Atlantic salmon (Salmo salar) to identify homeologous chromosome affinities within each species and ascertain homologous chromosome relationships among the species. Salmonid fishes exhibit a wide range of sex-specific differences in recombination rate, with some species having the largest differences for any vertebrate species studied to date. Our current estimate of female:male recombination rates in rainbow trout is 4.31:1. Chromosome structure and (or) size is associated with recombination rate differences between the sexes in rainbow trout. Linkage groups derived from presumptive acrocentric type chromosomes were observed to have much lower sex-specific differences in recombination rate than metacentric type linkage groups. Arctic charr is karyotypically the least derived species (i.e., possessing a high number of acrocentric chromosomes) and Atlantic salmon is the most derived (i.e., possessing a number of whole-arm fusions). Atlantic salmon have the largest female:male recombination ratio difference (i.e., 16.81:1) compared with rainbow trout, and Arctic charr (1.69:1). Comparisons of recombination rates between homologous segments of linkage groups among species indicated that when significant experiment-wise differences were detected (7/24 tests), recombination rates were generally higher in the species with a less-derived chromosome structure (6/7 significant comparisons). Greater similarity in linkage group syntenies were observed between Atlantic salmon and rainbow trout, suggesting their closer phylogenetic affinities, and most interspecific linkage group comparisons support a model that suggests whole chromosome arm translocations have occurred in the evolution of this group. However, some possible exceptions were detected and these findings are discussed in relation to their influence on segregation distortion patterns. We also report unusual meiotic segregation patterns in a female parent involving the duplicated (homeologous) linkage group pair 12/16 and discuss several models that may account for these patterns.