High Mutability in Male Hybrids of DROSOPHILA MELANOGASTER

The frequencies of sex-linked lethal mutations arising in hybrid male offspring from various crosses and in nonhybrid controls were determined. The hybrids were produced by crossing representative strains of the P-M system of hybrid dysgenesis in all possible combinations. Males from the cross of P males x M females had a mutation rate about 15 times higher than that of nonhybrid males from the P strain. Genetically identical males from the reciprocal cross had a mutation rate 3 to 4 times that of the nonhybrids. For crosses involving a Q strain, a significant increase in the mutation rate was detected in males produced by matings of Q males with M females. No increase was observed in genetically identical males from the reciprocal mating. Crosses between P and Q strains gave male hybrids with mutation rates not different from those of nonhybrids. Many of the lethals that occurred in hybrids from the cross of P males x M females appeared to be unstable; fewer lethals that arose in hybrids from the cross of Q males x M females were unstable. The relationship between P and Q strains is discussed with respect to a model of mutation induction in dysgenic hybrids.     

The genetic system controlling recombination in the silkworm

The nature of recombination modifiers was investigated in Bombyx mori lines selected for high (H) and low (L) recombination rates between the p^S and Y loci in chromosome 2. Since the mean recombination rates for the H x L and L x H F₁ crosses were approximately intermediate between those of high and low lines, the cytoplasmic maternal effect and difference in the activity of recombination modifiers between marked and unmarked second chromosomes were not detected. The H x (L x H), H x (H x L), L x (L x H) and L x (H x L) backcrosses indicated the presence of additive and dominance effects of marked and unmarked second chromosomes and the remaining chromosomes.——Recombination rates between the p^S and Y loci in chromosome 2 and half-nonrecombination rates between the pe and re loci in chromosome 5 of high and low lines indicated that these recombination modifiers caused changes in the recombination frequency between p^S and Y in chromosome 2, but not between pe and re in chromosome 5.——There were no differences in viability between individuals having the second chromosomes of the recombinant types [p^S +, p^Y (H); p^S +, + Y (L)] and those of the nonrecombinant types [p^S Y, p + (H); p^S Y, + + (L)] in both high and low lines. Mean recombination rates measured in cis [p^S Y/p + (H); p^S Y/+ + (L)] and trans [p^S +/p Y (H); p^S +/+ Y (L)] males were the same in the high but not in the low line. No segregation of a single recombination modifier was indicated by the distribution of recombination rates measured in trans males [p^S +/p Y (H); p^S +/+ Y (L)] of high and low lines. Accordingly, the recombination modifiers distributed on chromosome 2 in the heterozygous condition were not gross chromosomal aberrations, but polygenic factors in the low line.     

Genetic Mechanisms of Rare Matings of the Yeast SACCHAROMYCES CEREVISIAE Heterozygous for Mating Type

Yeast heterozygous for mating type lacks the ability to conjugate as judged by the mass-mating technique and accordingly is designated "non-mater". However, the non-mater shows rare mating ability with a frequency of less than 10^-6. In the present study, the RD auxotroph mating method was mainly employed with the intention of examining the rare mating ability of various non-maters, using lactate ethanol minimal medium as a selective medium for hybridization. Crosses of aα x a, aα x a, aaα x a, aαα x a, etc. resulted in the production of respective hybrids with a relatively high frequency of about 10^-6 to 10^-7, whereas crosses of aaα x a, aαα x α, aaαα x a, aaαα x α, etc. resulted in hybrids with an extremely low frequency of about less than 10^-8. Genetic analyses revealed that the rare matings were mostly caused by the presence of cells derived from the non-maters in which mating type had converted to a homozygous genotype. Mitotic recombination was shown to be a likely explanation for most of the conversion, judging from associated exchange of an outside marker, thr4. By successive employment of the RD auxotroph mating method, it was possible to produce a series of polyploid yeasts, triploids to octoploids. The DNA content and the cell volume were observed to increase parallel to the elevated ploidy states.     

Selective Recombination System in Bombyx mori. I. Chromosome Specificity of the Modification Effect

The effect of modifiers on recombination frequency between Ze and lem loci on chromosome 3 to elucidate the chromosome specificity of modification and the distribution of modifiers using Bombyx mori lines selected for high (H) and low (L) recombination rates between the p^S and Y loci in chromosome 2 was investigated. By crossing to the Z (Ze lem/++) line, the recombination rate between the p^S and Y loci in chromosome 2 was decreased from 28.18 to 23.33 in the H line and was increased from 4.92 to 16.05 in the L line. On the other hand, the recombination rate between the Ze and lem loci in chromosome 3 was increased from 16.21 to 20.21 in the Z line by crossing to the H line, but also increased to 19.02 by crossing to the L line. The significant correlation observed between the transformed recombination rates of chromosomes 2 and 3 in the (Z x L) x L backcross indicated that there were common factors modifying recombination frequency in chromosomes 2 and 3 or different factors linked to the same chromosomes. In the family of L x [(Z x L) x L] backcross, the distribution of transformed recombination rates indicated that there were several factors in the remaining chromosomes which were modifying recombination frequency in chromosome 2 but not in chromosome 3. It was also indicated that these factors were linked to different chromosomes than are the factors modifying recombination frequency in chromosome 3. In order to interpret these results, one genetic system model controlling recombination that consists of general and local recombination modifiers was proposed. The evolution of dynamic genetic systems that would effectively reduce recombinational load without reducing the advantage of recombination was discussed.     

A screen for recessive speciation genes expressed in the gametes of F1 hybrid yeast

Diploid hybrids of Saccharomyces cerevisiae and its closest relative, Saccharomyces paradoxus, are viable, but the sexual gametes they produce are not. One of several possible causes of this gamete inviability is incompatibility between genes from different species—such incompatible genes are usually called “speciation genes.” In diploid F1 hybrids, which contain a complete haploid genome from each species, the presence of compatible alleles can mask the effects of (recessive) incompatible speciation genes. But in the haploid gametes produced by F1 hybrids, recessive speciation genes may be exposed, killing the gametes and thus preventing F1 hybrids from reproducing sexually. Here I present the results of an experiment to detect incompatibilities that kill hybrid gametes. I transferred nine of the 16 S. paradoxus chromosomes individually into S. cerevisiae gametes and tested the ability of each to replace its S. cerevisiae homeolog. All nine chromosomes were compatible, producing nine viable haploid strains, each with 15 S. cerevisiae chromosomes and one S. paradoxus chromosome. Thus, none of these chromosomes contain speciation genes that were capable of killing the hybrid gametes that received them. This is a surprising result that suggests that such speciation genes do not play a major role in yeast speciation.     

Conditional Polygenic Effects in the Sternopleural Bristle System of DROSOPHILA MELANOGASTER

The chromosomal architecture of genotype x environment interactions was investigated in lines of Drosophila melanogaster selected for increased or decreased sternopleural bristle number at 18°, 25° and 29°. In general, interactions were found to have a stabilizing effect upon the bristle phenotype, in the sense that the genotype x environment interaction tended to increase bristle number under conditions in which temperature alone reduced bristle number and vice versa. The polygenic modifiers of mean bristle number were often separable from modifiers of the response to temperature both at the chromosomal level and intrachromosomally. In one of the low selection lines, a temperature-dependent polygenic locus was mapped on chromosome 3. It is suggested that genotype x environment interactions be thought of in terms of conditional polygenic expression. Such conditionality may be one of the ways in which polygenic variation is maintained in a population in the face of selection for an optimum phenotype.     

Odorant-binding proteins OBP57d and OBP57e affect taste perception and host-plant preference in Drosophila sechellia

Despite its morphological similarity to the other species in the Drosophila melanogaster species complex, D. sechellia has evolved distinct physiological and behavioral adaptations to its host plant Morinda citrifolia, commonly known as Tahitian Noni. The odor of the ripe fruit of M. citrifolia originates from hexanoic and octanoic acid. D. sechellia is attracted to these two fatty acids, whereas the other species in the complex are repelled. Here, using interspecies hybrids between D. melanogaster deficiency mutants and D. sechellia, we showed that the Odorant-binding protein 57e (Obp57e) gene is involved in the behavioral difference between the species. D. melanogaster knock-out flies for Obp57e and Obp57d showed altered behavioral responses to hexanoic acid and octanoic acid. Furthermore, the introduction of Obp57d and Obp57e from D. simulans and D. sechellia shifted the oviposition site preference of D. melanogaster Obp57d/e^KO flies to that of the original species, confirming the contribution of these genes to D. sechellia's specialization to M. citrifolia. Our finding of the genes involved in host-plant determination may lead to further understanding of mechanisms underlying taste perception, evolution of plant–herbivore interactions, and speciation.     

Incipient Genome Differentiation in Gossypium. II. Comparison of 12 Chromosomes in G. HIRSUTUM, G. MUSTELINUM and G. TOMENTOSUM Using Heterozygous Translocations

Eight homozygous translocation lines (TT) of G. hirsutum marking 3 chromosomes of the A genome and 9 chromosomes of the D genome were crossed with G. hirsutum, G. mustelinum and G. tomentosum, all homozygous for the standard end arrangements (tt). Chiasma frequencies in the G. hirsutum Tt controls were compared with those in the G. hirsutum x G. mustelinum and the G. hirsutum x G. tomentosum Tt hybrids. Both nucleus-wide and region-specific chiasma frequencies were compared.—Some genome differentiation appears to have arisen between G. hirsutum and G. mustelinum. The G. hirsutum x G. mustelinum hybrids had a 1.8 to 1.9% reduction in the nucleus-wide chiasma frequency. Four of the eight TT lines showed a 3.4 to 10.5% reduction in chiasmata in the hybrid translocation quadrivalents, suggesting that chromosomes 1, 21, 23 and 24 may have undergone localized genome differentiation. The two species may differ naturally in the end arrangement of two chromosomes, since a quadrivalent not due to experimentally introduced translocations was observed in 13% of the PMC's of two G. hirsutum x G. mustelinum hybrids.—Very little genome differentiation has occurred between G. hirsutum and G. tomentosum. In the G. hirsutum x G. tomentosum hybrids, the nucleus-wide estimates showed only a very small (0.1 to 0.2%), though statistically significant, lowering of the chiasma frequency, and there was no reduction in chiasma frequency in the more sensitive readings for specific translocation quadrivalents.     

Frequency of gamma-Ray Induced Specific Locus and Recessive Lethal Mutations in Mature Germ Cells of the Zebrafish, BRACHYDANIO RERIO

Specific locus and recessive lethal mutations are induced by γ-rays with approximately first order kinetics in the zebrafish (Brachydanio rerio) with frequencies of 4 x 10^-5 r^-1 and 4 x 10^-3 r^-1, respectively. The surprisingly low ratio (100:1) of recessive lethals to specific locus mutations may be due to the induction of large deficiencies by γ-rays.     

Population genomics: whole-genome analysis of polymorphism and divergence in Drosophila simulans

The population genetic perspective is that the processes shaping genomic variation can be revealed only through simultaneous investigation of sequence polymorphism and divergence within and between closely related species. Here we present a population genetic analysis of Drosophila simulans based on whole-genome shotgun sequencing of multiple inbred lines and comparison of the resulting data to genome assemblies of the closely related species, D. melanogaster and D. yakuba. We discovered previously unknown, large-scale fluctuations of polymorphism and divergence along chromosome arms, and significantly less polymorphism and faster divergence on the X chromosome. We generated a comprehensive list of functional elements in the D. simulans genome influenced by adaptive evolution. Finally, we characterized genomic patterns of base composition for coding and noncoding sequence. These results suggest several new hypotheses regarding the genetic and biological mechanisms controlling polymorphism and divergence across the Drosophila genome, and provide a rich resource for the investigation of adaptive evolution and functional variation in D. simulans.     

The genome sequence of Caenorhabditis briggsae: a platform for comparative genomics

The soil nematodes Caenorhabditis briggsae and Caenorhabditis elegans diverged from a common ancestor roughly 100 million years ago and yet are almost indistinguishable by eye. They have the same chromosome number and genome sizes, and they occupy the same ecological niche. To explore the basis for this striking conservation of structure and function, we have sequenced the C. briggsae genome to a high-quality draft stage and compared it to the finished C. elegans sequence. We predict approximately 19,500 protein-coding genes in the C. briggsae genome, roughly the same as in C. elegans. Of these, 12,200 have clear C. elegans orthologs, a further 6,500 have one or more clearly detectable C. elegans homologs, and approximately 800 C. briggsae genes have no detectable matches in C. elegans. Almost all of the noncoding RNAs (ncRNAs) known are shared between the two species. The two genomes exhibit extensive colinearity, and the rate of divergence appears to be higher in the chromosomal arms than in the centers. Operons, a distinctive feature of C. elegans, are highly conserved in C. briggsae, with the arrangement of genes being preserved in 96% of cases. The difference in size between the C. briggsae (estimated at approximately 104 Mbp) and C. elegans (100.3 Mbp) genomes is almost entirely due to repetitive sequence, which accounts for 22.4% of the C. briggsae genome in contrast to 16.5% of the C. elegans genome. Few, if any, repeat families are shared, suggesting that most were acquired after the two species diverged or are undergoing rapid evolution. Coclustering the C. elegans and C. briggsae proteins reveals 2,169 protein families of two or more members. Most of these are shared between the two species, but some appear to be expanding or contracting, and there seem to be as many as several hundred novel C. briggsae gene families. The C. briggsae draft sequence will greatly improve the annotation of the C. elegans genome. Based on similarity to C. briggsae, we found strong evidence for 1,300 new C. elegans genes. In addition, comparisons of the two genomes will help to understand the evolutionary forces that mold nematode genomes.     

Distribution and Frequency of Tandem Duplications of the rII Region of Bacteriophage T4d

Genetic analyses of 49 duplications of the rII region of bacteriophage T4D suggests that there is a non-random relationship between the end points of duplicated segments, that relaxed packaging restrictions have little if any effect on the distribution of duplications, that segregation is 3–4 times more frequent than normal recombination for the same interval, and that non-tandem duplications are rare. Extrapolation of the r1231 x rJ101 cross data suggests that the minimum frequency of duplications/genome is 1.7 x 10^-6, but possibly 3.4 x 10^-4.     

Mutation Rates for Enzyme and Morphological Loci in Barley ( HORDEUM VULGARE L.)

Spontaneous mutation rates were estimated by assaying 84,126 seedlings of a highly homozygous barley line (isogenic line 2025) for five enzyme loci. No mutants were observed in 841,260 allele replications. This result excludes, at probability level 0.95, a spontaneous mutation rate larger than 3.56 x 10^-6/locus/gamete/generation for these enzyme loci. Isogenic line 2025 also was scored for mutants at four loci governing morphological variants. No mutants were observed in 3,386,850 allele replications which indicates that the upper bound for the mutation rate for these loci is 8.85 x 10^-7. It was concluded that, even though spontaneous mutation has been important in creating variability in the barley species at the loci scored, the rate is too low to have much affect on the short-term dynamics of barley populations.     

The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains

Candida albicans has an elaborate, yet efficient, mating system that promotes conjugation between diploid a and α strains. The product of mating is a tetraploid a/α cell that must undergo a reductional division to return to the diploid state. Despite the presence of several “meiosis-specific” genes in the C. albicans genome, a meiotic program has not been observed. Instead, tetraploid products of mating can be induced to undergo efficient, random chromosome loss, often producing strains that are diploid, or close to diploid, in ploidy. Using SNP and comparative genome hybridization arrays we have now analyzed the genotypes of products from the C. albicans parasexual cycle. We show that the parasexual cycle generates progeny strains with shuffled combinations of the eight C. albicans chromosomes. In addition, several isolates had undergone extensive genetic recombination between homologous chromosomes, including multiple gene conversion events. Progeny strains exhibited altered colony morphologies on laboratory media, demonstrating that the parasexual cycle generates phenotypic variants of C. albicans. In several fungi, including Saccharomyces cerevisiae and Schizosaccharomyces pombe, the conserved Spo11 protein is integral to meiotic recombination, where it is required for the formation of DNA double-strand breaks. We show that deletion of SPO11 prevented genetic recombination between homologous chromosomes during the C. albicans parasexual cycle. These findings suggest that at least one meiosis-specific gene has been re-programmed to mediate genetic recombination during the alternative parasexual life cycle of C. albicans. We discuss, in light of the long association of C. albicans with warm-blooded animals, the potential advantages of a parasexual cycle over a conventional sexual cycle.     

Asymmetrical reinforcement and Wolbachia infection in Drosophila

Reinforcement refers to the evolution of increased mating discrimination against heterospecific individuals in zones of geographic overlap and can be considered a final stage in the speciation process. One the factors that may affect reinforcement is the degree to which hybrid matings result in the permanent loss of genes from a species' gene pool. Matings between females of Drosophila subquinaria and males of D. recens result in high levels of offspring mortality, due to interspecific cytoplasmic incompatibility caused by Wolbachia infection of D. recens. Such hybrid inviability is not manifested in matings between D. recens females and D. subquinaria males. Here we ask whether the asymmetrical hybrid inviability is associated with a corresponding asymmetry in the level of reinforcement. The geographic ranges of D. recens and D. subquinaria were found to overlap across a broad belt of boreal forest in central Canada. Females of D. subquinaria from the zone of sympatry exhibit much stronger levels of discrimination against males of D. recens than do females from allopatric populations. In contrast, such reproductive character displacement is not evident in D. recens, consistent with the expected effects of unidirectional cytoplasmic incompatibility. Furthermore, there is substantial behavioral isolation within D. subquinaria, because females from populations sympatric with D. recens discriminate against allopatric conspecific males, whereas females from populations allopatric with D. recens show no discrimination against any conspecific males. Patterns of general genetic differentiation among populations are not consistent with patterns of behavioral discrimination, which suggests that the behavioral isolation within D. subquinaria results from selection against mating with Wolbachia-infected D. recens. Interspecific cytoplasmic incompatibility may contribute not only to post-mating isolation, an effect already widely recognized, but also to reinforcement, particularly in the uninfected species. The resulting reproductive character displacement not only increases behavioral isolation from the Wolbachia-infected species, but may also lead to behavioral isolation between populations of the uninfected species. Given the widespread occurrence of Wolbachia among insects, it thus appears that there are multiple ways by which these endosymbionts may directly and indirectly contribute to reproductive isolation and speciation.     

Chloroplast and Mitochondrial DNA Variation in HORDEUM VULGARE and HORDEUM SPONTANEUM

A survey of restriction fragment polymorphism in Hordeum vulgare and Hordeum spontaneum was made using 17 and 16 hexanucleotide restriction endonucleases on chloroplast (cp) and mitochondrial (mt) DNA, respectively. The plant accessions originated from various places throughout the Fertile Cresent and Mediterranean. The types of changes in cpDNA consisted of nucleotide substitutions and insertions and deletions on the order of 100 base pairs. In contrast, mtDNA has most likely undergone larger insertions and deletions of up to 20 kilobase pairs in addition to rearrangements. Grouping of mtDNA fragment data showed that in some cases geographical affinities existed between the two species, whereas in others there were no clear affinities. Nucleotide diversity estimates derived from the restriction fragment data were used in a number of comparisons of variability. Comparisons of overall mtDNA variability (nucleotide diversity = 9.68 x 10^-4) with cpDNA variability (nucleotide diversity = 6.38 x 10^-4 ) indicated that the former are somewhat more variable. Furthermore, there was no indication that the wild H. spontaneum (cpDNA diversity = 5.57 x 10^-4; mtDNA diversity = 6.04 x 10 ^-4) was more variable than the land races of H. vulgare (cpDNA diversity = 5.88 x 10^-4; mtDNA diversity = 9.79 x 10^-4). In fact, on the basis of mtDNA diversity, H. vulgare was the more variable species. Comparison of organelle nucleotide diversity estimates with an estimate of nuclear nucleotide diversity derived from existing isozyme data provided evidence that both organelle genomes are evolving at a slower rate than the nuclear genome.     

The complete genome sequence and comparative genome analysis of the high pathogenicity Yersinia enterocolitica strain 8081

The human enteropathogen, Yersinia enterocolitica, is a significant link in the range of Yersinia pathologies extending from mild gastroenteritis to bubonic plague. Comparison at the genomic level is a key step in our understanding of the genetic basis for this pathogenicity spectrum. Here we report the genome of Y. enterocolitica strain 8081 (serotype 0:8; biotype 1B) and extensive microarray data relating to the genetic diversity of the Y. enterocolitica species. Our analysis reveals that the genome of Y. enterocolitica strain 8081 is a patchwork of horizontally acquired genetic loci, including a plasticity zone of 199 kb containing an extraordinarily high density of virulence genes. Microarray analysis has provided insights into species-specific Y. enterocolitica gene functions and the intraspecies differences between the high, low, and nonpathogenic Y. enterocolitica biotypes. Through comparative genome sequence analysis we provide new information on the evolution of the Yersinia. We identify numerous loci that represent ancestral clusters of genes potentially important in enteric survival and pathogenesis, which have been lost or are in the process of being lost, in the other sequenced Yersinia lineages. Our analysis also highlights large metabolic operons in Y. enterocolitica that are absent in the related enteropathogen, Yersinia pseudotuberculosis, indicating major differences in niche and nutrients used within the mammalian gut. These include clusters directing, the production of hydrogenases, tetrathionate respiration, cobalamin synthesis, and propanediol utilisation. Along with ancestral gene clusters, the genome of Y. enterocolitica has revealed species-specific and enteropathogen-specific loci. This has provided important insights into the pathology of this bacterium and, more broadly, into the evolution of the genus. Moreover, wider investigations looking at the patterns of gene loss and gain in the Yersinia have highlighted common themes in the genome evolution of other human enteropathogens.     

Rare-Cell Fusion Events between Diploid and Haploid Strains of the Sexually Agglutinative Yeast HANSENULA WINGEI

Diploids of the yeast Hansenula wingei are nonagglutinative and do not form zygotes in mixed cultures with either sexually agglutinative haploid mating type. However, a low frequency of diploid x haploid cell fusions (about 10^-3) is detectable by prototrophic selection. This frequency of rare diploid x haploid matings is not increased after the diploid culture is induced for sexual agglutination. Therefore, we conclude that genes that repress mating are different from those that repress sexual agglutination.——Six prototrophs isolated from one diploid x haploid cross had an average DNA value (µg DNA per 10⁸ cells) of 6.19, compared to 2.53 and 4.35 for the haploid and diploid strains, respectively. Four prototrophs were clearly cell-fusion products because they contained genes from both the diploid and the haploid partners. However, genetic analysis of the prototrophs yielded results inconsistent with triploid meiosis; all six isolates yielded a 2:2 segregation for the mating-type alleles and linked genes.——Mitotic segregation of monosomic (2n-1) cells lacking one homolog of the chromosome carrying the mating-type locus is proposed to explain the rare production of sexually active cells in the diploid cultures. Fusion between such monosomic cells and normal haploids is thought to have produced 3n-1 cells, disomic for the chromosome carrying the mating-type locus. We conclude that in the diploid strain we studied, the physiological mechanisms repressing sexual agglutination and conjugation function efficiently, but events occuring during mitosis lead to a low frequency of genetically altered cells in the population.     

Major structural differences and novel potential virulence mechanisms from the genomes of multiple campylobacter species

Sequencing and comparative genome analysis of four strains of Campylobacter including C. lari RM2100, C. upsaliensis RM3195, and C. coli RM2228 has revealed major structural differences that are associated with the insertion of phage- and plasmid-like genomic islands, as well as major variations in the lipooligosaccharide complex. Poly G tracts are longer, are greater in number, and show greater variability in C. upsaliensis than in the other species. Many genes involved in host colonization, including racR/S, cadF, cdt, ciaB, and flagellin genes, are conserved across the species, but variations that appear to be species specific are evident for a lipooligosaccharide locus, a capsular (extracellular) polysaccharide locus, and a novel Campylobacter putative licABCD virulence locus. The strains also vary in their metabolic profiles, as well as their resistance profiles to a range of antibiotics. It is evident that the newly identified hypothetical and conserved hypothetical proteins, as well as uncharacterized two-component regulatory systems and membrane proteins, may hold additional significant information on the major differences in virulence among the species, as well as the specificity of the strains for particular hosts.     

Complex genetic interactions in a quantitative trait locus

Whether in natural populations or between two unrelated members of a species, most phenotypic variation is quantitative. To analyze such quantitative traits, one must first map the underlying quantitative trait loci. Next, and far more difficult, one must identify the quantitative trait genes (QTGs), characterize QTG interactions, and identify the phenotypically relevant polymorphisms to determine how QTGs contribute to phenotype. In this work, we analyzed three Saccharomyces cerevisiae high-temperature growth (Htg) QTGs (MKT1, END3, and RHO2). We observed a high level of genetic interactions among QTGs and strain background. Interestingly, while the MKT1 and END3 coding polymorphisms contribute to phenotype, it is the RHO2 3′UTR polymorphisms that are phenotypically relevant. Reciprocal hemizygosity analysis of the Htg QTGs in hybrids between S288c and ten unrelated S. cerevisiae strains reveals that the contributions of the Htg QTGs are not conserved in nine other hybrids, which has implications for QTG identification by marker-trait association. Our findings demonstrate the variety and complexity of QTG contributions to phenotype, the impact of genetic background, and the value of quantitative genetic studies in S. cerevisiae.