Identification of two loci for resistance to clubroot (<Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis> Woronin) in <Emphasis Type="Italic">Brassica rapa</Emphasis> L.

In an analysis of 114 F₂ individuals from a cross between clubroot-resistant and susceptible lines of Brassica rapa L., 'G004' and 'Hakusai Chukanbohon Nou 7' (A9709), respectively, we identified two loci, Crr1 and Crr2, for clubroot (caused by Plasmodiophora brassicae Woronin) resistance. Each locus segregated independently among the F₂ population, indicating that the loci reside on a different region of chromosomes or on different chromosomes. Genetic analysis showed that each locus had little effect on clubroot resistance by itself, indicating that these two loci are complementary for clubroot resistance. The resistance to clubroot was much stronger when both loci were homozygous for resistant alleles than when they were heterozygous. These results indicate that clubroot resistance in B. rapa is under oligogenic control and at least two loci are necessary for resistance.Communicated by H.C. Becker     

Quantitative trait locus analysis in crosses between outbred lines with dominance and inbreeding

In an effort to determine the genetic basis of exceptionally large tomato fruits, QTL analysis was performed on a population derived from a cross between the wild species Lycopersicon pimpinellifolium (average fruit weight, 1 g) and the L. esculentum cultivar var. Giant Heirloom, which bears fruit in excess of 1000 g. QTL analysis revealed that the majority (67%) of phenotypic variation in fruit size could be attributed to six major loci localized on chromosomes 1-3 and 11. None of the QTL map to novel regions of the genome-all have been reported in previous studies involving moderately sized tomatoes. This result suggests that no major QTL beyond those already reported were involved in the evolution of extremely large fruit. However, this is the first time that all six QTL have emerged in a single population, suggesting that exceptionally large-fruited varieties, such as Giant Heirloom, are the result of a novel combination of preexisting QTL alleles. One of the detected QTL, fw2.2, has been cloned and exerts its effect on fruit size through global control of cell division early in carpel/fruit development. However, the most significant QTL detected in this study (fw11.3, lcn11.1) maps to the bottom of chromosome 11 and seems to exert its effect on fruit size through control of carpel/locule number. A second major locus, also affecting carpel number (and hence fruit size), was mapped to chromosome 2 (fw2.1, lcn2.1). We propose that these two carpel number QTL correspond to the loci described by early classical geneticists as fasciated (f) and locule number (lc), respectively.     

The making of a bell pepper-shaped tomato fruit: identification of loci controlling fruit morphology in Yellow Stuffer tomato

The heirloom tomato cultivar Yellow Stuffer produces fruit that are similar in shape and structure to fruit produced by the bell pepper varieties of garden pepper. To determine the genetic basis of this extreme fruit type in tomato, quantitative trait loci (QTL) analysis was performed on an F(2) population derived from a cross between Yellow Stuffer and the related species, Lycopersicon pimpinellifolium, which produces a small, round fruit typical of most wild species. F(2) plants were analyzed for both fruit size and the degree to which their fruit resembled the bell pepper. Three QTL were determined to influence bell pepper shape and seven QTL influenced fruit mass. The map positions of all three bell shape and six out of seven fruit size QTL appear to be allelic to components of fruit morphology analyzed in this population and to major fruit morphology QTL reported previously, adding support to the hypothesis that the majority of fruit size and shape variation in cultivated tomato is attributable to allelic variation at a limited number of loci. However, novel loci controlling components of fruit morphology, such as elongated fruit shape, bumpiness, number of seed per fruit and flowers per inflorescence were identified in this study as well. The three bell shape loci involved are: bell2.1, bell2.2 and bell8.1, and appear to correspond to locule number2.1 ( lcn2.1) and fruit weight 2.2 ( fw2.2) and fruit shape 8.1 ( fs8.1), respectively. The Yellow Stuffer alleles at lcn2.1 and fw2.2 increase locule number and fruit size, respectively, hence contributing to the overall bell pepper shape. The Yellow Stuffer allele at fs8.1 causes convex locule walls, giving the extended, bumpy shape characteristic of bell peppers. In addition, most fruit size QTL correspond to loci controlling number of flowers per inflorescence and/or stem-end blockiness. Comparisons among previously identified fruit morphology loci in tomato, eggplant and pepper suggest that loci affecting several aspects of fruit morphology may be due to pleiotrophic effects of the same, orthologous loci in these species. Moreover, it appears that the evolution of bell pepper-shaped tomato fruit may have proceeded through mutations of some of the same genes that led to bell pepper-type fruit in garden pepper.     

Genetic studies of histocompatibility in rabbits: Identification of major and minor genes

Histocompatibility was investigated by skin grafting in F₂ and F₃ hybrid generations from a cross of two related inbred lines of rabbits. The results suggested that two loci (RbH- 1, RbH- 2), in addition to a number of minor loci contributing weak effects on histocompatibility, are segregating in these generations.RbH- 1 may correspond or be homologous to the complex locus found in a number of mammalian species.     

Chromosomal inheritance of parental rDNAs distribution pattern detected by FISH in diploid F1 hybrid progeny of Cobitis (Teleostei,Cobitidae) species has non-Mendelian character

This study was conducted to describe the major and the minor rDNA chromosome distribution in the spined loach Cobitis taenia (2n = 48) and the Danubian loach Cobitis elongatoides (2n = 50), and their laboratory-produced diploid reciprocal F₁ hybrid progeny. It was tested by fluorescence in situ hybridisation (FISH) whether the number of 28s and 5s rDNA sites in the karyotypes of diploid hybrids corresponds to the expectations resulting from Mendelian ratio and if nucleolar organiser regions (NOR)were inherited from both parents or nucleolar dominance can be observed in the induced F₁ hybrid progeny. Ten (females) or twelve (males) 28s rDNA loci were located in nine uniarm chromosomes of C. taenia. Two of such loci terminally bounded on one acrocentric chromosome were unique and indicated as specific for this species. Large 5s rDNA clusters were located on two acrocentric chromosomes. In C. elongatoides of both sexes, six NOR sites in terminal regions on six meta-submetacentric chromosomes and two 5s rDNA sites on large submetacentrics were detected. The F₁ hybrid progeny (2n = 49) was characterised by the intermediate karyotype with the sites of ribosome synthesis on chromosomes inherited from both parents without showing nucleolar dominance. 5s rDNA sites were detected on large submetacentric and two acrocentric chromosomes. The observed number of both 28s and 5s rDNAs signals in F₁ diploid Cobitis hybrids was disproportionally inherited from the two parental species, showing inconsistency with the Mendelian ratios. The presented rDNA patterns indicate some marker chromosomes that allow the species of the parental male and female to be recognised in hybrid progeny. The 5s rDNA was found to be a particularly effective diagnostic marker of C. elongatoides to partially discern genomic composition of diploid Cobitis hybrids and presumably allopolyploids resulting from their backcrossing with one of the parental species. Thus, the current study provides insight into the extent of rDNA heredity in Cobitis chromosomes and their cytotaxonomic character.     

On growth, fatness, and form: A further look at porcine Chromosome 4 in an Iberian × Landrace cross

A crossed population between Iberian × Landrace pigs consisting of 321 F₂, 87 F₃, and 85 backcross individuals has been analyzed to refine the number and positions of quantitative trait loci (QTL) affecting shape, growth, fatness, and meat quality traits in SSC4. A multitrait multi-QTL approach has been used. Our results suggest that carcass length and shoulder weight are affected by two loci. The first one, close to the AFABP gene, has a very strong pleiotropic effect on fatness, whereas the second one, in the interval between S0073 and S0214, also affects live weight, although to a lesser extent. This latter QTL would correspond to the FAT1 locus described initially in pigs. It seems that SSC4’s loci play an important role in redistributing total weight, and the Landrace allele increases shoulder weight and carcass length much more than ham or total weight. Furthermore, there is also strong evidence of additional loci influencing pH and color in more distant, telomeric positions.     

Genetic analysis of tolerance to low-phosphorus stress in maize using restriction fragment length polymorphisms

Summary An understanding of the genetic nature underlying tolerance to low-phosphorus (low-P) stress could aid in the efficient development of tolerant plant strains. The objective of this study was to identify the number of loci in a maize (Zea mays L.) population segregating for tolerance to low-P stress, their approximate location, and the magnitude of their effect.Seventy-seven restriction fragment length polymorphisms (RFLPs) were identified and scored in a maize F₂ population derived from a cross between line NY821 and line H99. The F₂ individuals were self-pollinated to produce F₃ families. Ninety F₃ families were grown in a sand-alumina system, which simulated diffusion-limited, low-P soil conditions. The F₃ families were evaluated for vegetative growth in a controlled-environment experiment. To identify quantitative trait loci (QTLs) underlying tolerance to low-P stress, the mean phenotypic performances of the F₃ families were contrasted based on genotypic classification at each of 77 RFLP marker loci.Six RFLP marker loci were significantly associated with performance under low-P stress (P<0.01). One marker locus accounted for 25% of the total phenotypic variation. Additive gene action was predominant for all of the QTLs identified. Significant marker loci were located on four separate chromosomes representing five unlinked genomic regions. Two marker loci were associated with an additive by additive epistatic interaction. A multiple regression model including three marker loci and the significant epistatic interaction accounted for 46% of the total phenotypic variation. Heterozygosity per se was not predictive of phenotypic performance.     

Chromosomal location of genes conditioning low amylose content of endosperm starches in rice,Oryza sativa L.

Summary Eight dull mutants that lower the amylose content of rice endosperm as well as waxy mutant and a cultivar with common grains were crossed in a diallele manner. The amylose content of F₁ and F₂ seeds was determined on the basis of single grain analysis. It was concluded that the low amylose content of dull mutants is under monogenic recessive control. Alleles for low amylose content are located at five loci designated as du-1, du-2, du-3, du-4 and du-5. These loci are independent of wx locus located on chromosome 6. The five du loci have an additive effect in lowering the amylose content. Two loci, du-1 and du-4, were found to be located on chromosomes 7 and 4, respectively.     

Molecular marker analyses of powdery mildew resistance in barley

Powdery mildew, caused byEryisphe graminis f. sp.hordei, is one of the most important diseases of barley (Hordeum vulgare). A number of loci conditioning resistance to this disease have been reported previously. The objective of this study was to use molecular markers to identify chromosomal regions containing genes for powdery mildew resistance and to estimate the resistance effect of each locus. A set of 28 F₁ hybrids and eight parental lines from a barley diallel study was inoculated with each of five isolates ofE. graminis. The parents were surveyed for restriction fragment length polymorphisms (RFLPs) at 84 marker loci that cover about 1100 cM of the barley genome. The RFLP genotypes of the F₁s were deduced from those of the parents. A total of 27 loci, distributed on six of the seven barley chromosomes, detected significant resistance effects to at least one of the five isolates. Almost all the chromosomal regions previously reported to carry genes for powdery mildew resistance were detected, plus the possible existence of 1 additional locus on chromosome 7. The analysis indicated that additive genetic effects are the most important component in conditioning powdery mildew resistance. However, there is also a considerable amount of dominance effects at most loci, and even overdominance is likely to be present at a number of loci. These results suggest that quantitative differences are likely to exist among alleles even at loci which are considered to carry major genes for resistance, and minor effects may be prevalent in cultivars that are not known to carry major genes for resistance.     

Validated quantitative trait loci for eggshell quality in experimental and commercial laying hens

Compromised eggshell quality causes considerable economic losses for the egg industry. Breeding for improved eggshell quality has been very challenging. Eggshell quality is a trait that would greatly benefit from marker‐assisted selection, which would allow the selection of sires for their direct contribution to the trait and would also allow implementation of measurements integrating a number of shell parameters that are difficult to measure. In this study, we selected the most promising autosomal quantitative trait loci (QTL) affecting eggshell quality on chromosomes 2, 3, 6 and 14 from earlier experiments and we extended the F₂ population to include 1599 F₂ females. The study was repeated on two commercial populations: Lohmann Tierzucht Rhode Island Red line (n = 692 females) and a Hy‐Line White Plymouth Rock line (n = 290 progeny tested males). We analyzed the selected autosomal QTL regions on the three populations with SNP markers at 4–13 SNPs/Mb density. QTL for eggshell quality were replicated on all studied regions in the F₂ population. New QTL were detected for eggshell color on chromosomes 3 and 6. Marker associations with eggshell quality traits were validated in the tested commercial lines on chromosomes 2, 3 and 6, thus paving the way for marker‐assisted selection for improved eggshell quality.     

Mapping quantitative trait loci associated with regeneration ability of seed callus in rice, Oryza sativa L.

 Quantitative trait loci (QTL) controlling the regeneration ability of rice seed callus were detected using 245 RFLP markers and 98 BC₁F₅ lines derived from two varieties, ‘Nipponbare’ and ‘Kasalath’. Regeneration ability was evaluated by two indices: average number of regenerated shoots per callus (NRS) and regeneration rate (RR). The BC₁F₅ lines showed continuous segregation for both indices. Five putative QTL for NRS (tentatively named qRg1, qRg2, qRg4a, qRg4b and qRg4c) located on chromosomes 1, 2 and 4 were detected. Digenic interaction among these detected QTL was not significant (P<0.01). Among the five QTL detected, four ‘Kasalath’ alleles and one ‘Nipponbare’ allele increased NRS. According to an estimate based on the nearest marker loci, the five QTL accounted for 38.5% of the total phenotypic variation of the BC₁F₅ lines. For RR, four putative QTL were detected on chromosomes 2 and 4, and all of these were in the same chromosomal regions as the NRS QTL. The four RR QTL accounted for 32.6% of the total phenotypic variation. Received: 7 November 1996 / Accepted: 25 April 1997     

Molecular mapping of the centromeres of tomato chromosomes 7 and 9

The centromeres of two tomato chromosomes have been precisely localized on the molecular linkage map through dosage analysis of trisomic stocks. To map the centromeres of chromosomes 7 and 9, complementary telo-, secondary, and tertiary trisomic stocks were used to assign DNA markers to their respective chromosome arms and thus to localize the centromere at the junction of the short and long arms. It was found that both centromeres are situated within a cluster of cosegregating markers. In an attempt to order the markers within the centric clusters, genetic maps of the centromeric regions of chromosomes 7 and 9 were constructed from F₂ populations of 1620Lycopersicon esculentum × L. pennellii (E × P) plants and 1640L. esculentum × L. pimpinellifolium (E × PM) plants. Despite the large number of plants analyzed, very few recombination events were detected in the centric regions, indicating a significant suppression of recombination at this region of the chromosome. The fact that recombination suppression is equally strong in crosses between closely related (E × PM) and remotely related (E × P) parents suggests that centromeric suppression is not due to DNA sequence mismatches but to some other mechanism. The greatest number of centromeric markers was resolved in theL. esculentum × L. pennellii F₂ population. The centromere of chromosome 7 is surrounded by eight cosegregating markers: three on the short arm, five on the long arm. Similarly, the centric region of chromosome 9 contains ten cosegregating markers including one short arm marker and nine long arm markers. The localization of centromeres to precise intervals on the molecular linkage map represents the first step towards the characterization and ultimate isolation of tomato centromeres.     

Multivariate analysis of quantitative trait loci influencing variation in anxiety-related behavior in laboratory mice

The number and mode of action of quantitative trait loci (QTL) that contribute to behavioral variation in rodents is still largely unknown. On theoretical grounds, multivariate techniques are expected to yield new insights into this problem, but there are only a few examples of its application in practice. Here we explore the power of multivariate approaches to uncover the genetic architecture of 23 anxiety-related phenotypes in 1636 F₂ laboratory mice. We detected QTL with a genome-wide significance threshold of P < 0.05 on 14 chromosomes, of which 10 correspond to those identified by univariate analysis. Novel QTL were found on Chromosomes 3, 9, 13, and 17. Thus, multivariate analyses increased the yield of QTL exceeding a genome-wide significance threshold by 40%. On the basis of these results and by the application of a QTL estimator, we show that the mean number of QTL influencing anxiety-related behavior in mice is 6, with a 95% upper limit of 14.     

Inheritance of Arabidopsis DNA in offspring from Brassica napus and A. thaliana somatic hybrids

 Chromosome counts and RFLP markers mapped to Arabidopsis thaliana were used to determine the proportion of eliminated chromosomes and retained A. thaliana DNA in the back-crossed (BC) progeny derived from symmetric and asymmetric somatic hybrids between Brassica napus and A. thaliana. All plants were analysed for the presence of two RFLP markers per chromosome, preferably with one located on each chromosome arm. A reduction in both A. thaliana RFLP markers and chromosome numbers was found in the BC₁ and BC₂ generations of the symmetric hybrids as well as in the BC₁ generation of the asymmetric hybrids. In the symmetric hybrids, two back-crosses to B. napus were required to reduce the frequency of retained A. thaliana loci to 42.4% and mean chromosome number to 39.4. In comparison, the BC₁ progeny of the asymmetric hybrids had 16% of the analysed A. thaliana loci present and an average of 38.4 chromosomes maintained. When the frequency of A. thaliana chromosomes with both analysed loci maintained was compared with the frequency of chromosomes with one locus lost and one kept, a reduction in the number of complete chromosomes between BC₁ and BC₂ derived from the symmetric hybrids was observed. Among the BC₁ plants in the asymmetric group the situation was different, with higher amounts of incomplete donor chromosomes compared to whole chromosomes. The results indicate that A. thaliana chromosome fragments are more often found in the progeny of irradiated hybrids, while back-crossed symmetric hybrids have more complete chromosomes. Received: 2 April 1998 / Accepted: 14 July 1998     

Estimations of number of active loci,dominance and heritability in polygenic inheritance of gregarious behavior inMuscidifurax raptorellus [Hymenoptera: Pteromalidae]

Estimates of the minimum number of genetic loci governing expressions of gregarious oviposition inMuscidifurax raptorellus Kogan & Legner were enabled by the verification of recombinant males, which were secured from virgin hybrid females, formed by crossing cohorts from solitary and gregarious populations. Examinations of parasitization behavior in female progeny that had F₁ male fathers indicated the presence of hybrids among the males. Estimates of gene number, made on the basis of variances in P₁, F₁, F₂ and backcross progeny, and by observing behavior in second and third order backcrosses, ranged from two to 19, with most between two and five. However, backcrossing data suggested that at least eight loci were actively segregating for this characteristic. Semi-dominance of the solitary trait (D=0.63 to 0.84), and unequal gene effects probably caused these gene estimated to be lower than the actual number. Estimates of the coefficient of heritability, in the broad sense based on parental and F₁ and F₂ variances indicated that variability of gregarious behavior in the experimental environment was influenced >60% by genotypic factors, offspring-parent regression analyses gave estimates >38%.      

Cytogenetic studies on natural intergeneric hybridization inAster alliances

The karyotype and meiosis of the 12-ploid plants—one of the offspring of the natural F₁ hybrid (Aster ageratoides subsp.ovatus (2n=36) ×Kalimeris incisa (2n=72), 2n=72)—were examined. The 2n=108 chromosomes of the 12-ploids were found to consist of 18 large chromosomes and 90 small chromosomes. In meiosis of the PMCs of the 12-ploid, chromosome configurations of 3_III+46_II+7_I, 2_III+48_II+6_I and 3_III+47_II+5_I were observed. All the univalents and trivalents were small, and among the 46–48 bivalents nine were large and the remaining 37–39 were comparatively small. The large bivalents were found to represent autosyndetic pairings, and the small bivalents and trivalents were probably formed by autosyndetic pairings. The large chromosomes of the 12-ploids were found to coincide with the large chromosomes ofovatus, and the 90 small chromosomes to correspond to small chromosomes ofovatus andK. incisa. The 12-ploids were concluded to have been produced by a fusion of an unreduced gamete of the F₁ plant and a reduced gamete ofK. incisa which was growing in proximity to the F₁s. Thus the 12-ploids were regarded to be an amphidiploid having 36 chromosomes ofovatus and 72 chromosomes ofK. incisa.     

Genome balance in six successive generations of the allotetraploid Festuca pratensis × Lolium perenne

In the allotetraploid, Festuca pratensis Huds. (2n = 4x = 28) × Lolium perenne L. (2n = 4x = 28) the balance of chromatin, as determined by GISH, changes over successive generations of open pollination in favour of L. perenne. There is extensive recombination between chromosomes of the two parental genomes, as well as substitution of whole Festuca chromosomes by whole Lolium chromosomes. The total number of Lolium chromosomes increased from a mean 14.36 in the F₂ to 16.26 in the F₆, and the total number of Festuca chromosomes decreased correspondingly from a mean of 13.57 to a value of 11.56. The number of recombinant chromosomes and recombination breakpoints per genotype also increased from generation to generation, although the respective values of both characters were higher for Festuca (0.86–8.41 and 1.14–15.22) than for Lolium (0.68–4.59 and 0.68–6.0). The proportion of total genome length contributed by the L. perenne chromatin increased from about 50% in F₂ to 59.5% in F₆. The results are based on the sample of 134 plants studied (26–28 plants per generation), and are discussed in terms of the dominance of Lolium chromosomes over those of Festuca, and possible mechanisms underlying this phenomenon of chromatin substitution.     

Identification of quantitative trait loci controlling resistance to maize chlorotic dwarf virus

Ineffective screening methods and low levels of disease resistance have hampered genetic analysis of maize (Zea mays L.) resistance to disease caused by maize chlorotic dwarf virus (MCDV). Progeny from a cross between the highly resistant maize inbred line Oh1VI and the susceptible inbred line Va35 were evaluated for MCDV symptoms after multiple virus inoculations, using the viral vector Graminella nigrifrons. Symptom severity scores from three rating dates were used to calculate area under the disease progress curve (AUDPC) scores for vein banding, leaf twist and tear, and whorl chlorosis. AUDPC scores for the F₂ population indicated that MCDV resistance was quantitatively inherited. Genotypic and phenotypic analyses of 314 F₂ individuals were compared using composite interval mapping (CIM) and analysis of variance. CIM identified two major quantitative trait loci (QTL) on chromosomes 3 and 10 and two minor QTL on chromosomes 4 and 6. Resistance was additive, with alleles from Oh1VI at the loci on chromosomes 3 and 10 contributing equally to resistance.     

Molecular-marker-facilitated investigation on the ability to stimulate N2 fixation in the rhizosphere by irrigated rice plants

An F₂ population, consisting of 231 individuals derived from a cross between rice cultivars with a similar growing duration, Palawan and IR42, was utilized to investigate the genetic nature of rice varietal ability to stimulate N₂ fixation in the rice rhizosphere. To assess rhizospheric N₂ fixation, an isotope-enriched ¹⁵N dilution technique was employed, using ¹⁵N-stabilized soil in pots. IR42, an indica variety, had 23% higher N derived from fixation (Ndfa) than Palawan, a javanica genotype. Normal segregation of atom% ¹⁵N excess was obtained in the F₂ population, with an average of 0.218 with 8% of plants below IR42 (0.188) and 10% of plants above Palawan (0.248). One-hundred-and-four RFLP markers mapped on 12 chromosomes were tested for linkage to the putative QTLs. Significant (P<0.01) associations between markers and segregation of atom% ¹⁵N excess were observed for seven marker loci located on chromosomes 1, 3, 6 and 11. Four QTLs defined by the detected marker loci were identified by interval-mapping analysis. Additive gene action was found to be predominant, but for at least one locus, dominance and partial dominance effects were observed. Significant (P<0.01) epistatic effects were also identified. Individual marker loci detected between 8 and 16% of the total phenotypic variation. All four putative QTLs showed recessive gene action, and no phenotypic effects associated with heterozygosity of marker loci were observed. The results of this study suggest that rice genetic factors can be identified which affect levels of atom% ¹⁵N excess in the soil by interacting with diazotrophs in the rice rhizosphere.     

Population structure of nuclear and mitochondrial DNA variation among humpback whales in the North Pacific

The population structure of variation in a nuclear actin intron and the control region of mitochondrial DNA is described for humpback whales from eight regions in the North Pacific Ocean: central California, Baja Peninsula, nearshore Mexico (Bahia Banderas), offshore Mexico (Socorro Island), southeastern Alaska, central Alaska (Prince Williams Sound), Hawaii and Japan (Ogasawara Islands). Primary mtDNA haplotypes and intron alleles were identified using selected restriction fragment length polymorphisms of target sequences amplified by the polymerase chain reaction (PCR–RFLP). There was little evidence of heterogeneity in the frequencies of mtDNA haplotypes or actin intron alleles due to the year or sex composition of the sample. However, frequencies of four mtDNA haplotypes showed marked regional differences in their distributions (Φ_ST = 0.277; P < 0.001; n = 205 individuals) while the two alleles showed significant, but less marked, regional differences (Φ_ST = 0.033; P < 0.013; n = 400 chromosomes). An hierarchical analysis of variance in frequencies of haplotypes and alleles supported the grouping of six regions into a central and eastern stock with further partitioning of variance among regions within stocks for haplotypes but not for alleles. Based on available genetic and demographic evidence, the southeastern Alaska and central California feeding grounds were selected for additional analyses of nuclear differentiation using allelic variation at four microsatellite loci. All four loci showed significant differences in allele frequencies (overall F_ST = 0.043; P < 0.001; average n = 139 chromosomes per locus), indicating at least partial reproductive isolation between the two regions as well as the segregation of mtDNA lineages. Although the two feeding grounds were not panmictic for nuclear or mitochondrial loci, estimates of long-term migration rates suggested that male-mediated gene flow was several-fold greater than female gene flow. These results include and extend the range and sample size of previously published work, providing additional evidence for the significance of genetic management units within oceanic populations of humpback whales.