Male F1 Hybrids Research Articles

Measurements of hybrid fertility and a test of mate preference for two house mouse races with massive chromosomal divergence

BackgroundWestern house mice Mus musculus domesticus are among the most important mammalian model species for chromosomal speciation. Hybrids between chromosomal races of M. m. domesticus suffer various degrees of fertility reduction between full fertility and complete sterility, depending on the complexity of the chromosomal differences between the races. This complexity presents itself in hybrids as meiotic configurations of chromosome chains and rings, with longer configurations having a stronger impact on fertility. While hybrids with short configurations have been intensively studied, less work has been done on hybrids with very long configurations. In this study, we investigated laboratory-reared wild mice from two chromosomally very different races in Switzerland found in close proximity. Hybrids between these races form a meiotic chain of fifteen chromosomes. We performed a detailed analysis of male and female hybrid fertility, including three generations of female backcrosses to one of the parental races. We also tested for possible divergence of mate preference in females.ResultsWhile all male F1 hybrids were sterile with sperm counts of zero, 48% of female F1 hybrids produced offspring. Their litter sizes ranged from one to three which is significantly lower than the litter size of parental race females. When hybrid females were backcrossed to a parental race, half of the offspring resembled the parental race in karyotype and fertility, while the other half resembled the F1 hybrids. In the preference test, females of both races indicated a lack of a preference for males of their own karyotype.ConclusionsAlthough the fertility of the F1 hybrids was extremely low because of the complexity of the chromosomal differences between the races, reproductive isolation was not complete. As we did not find assortative female preferences, we expect that contact between these races would lead to the production of hybrids and that gene flow would occur eventually, as fertility can be restored fully after one backcross generation.

Viability and fertility of hybrid New Zealand tree wētā Hemideina spp. (Orthoptera: Anostostomatidae)

Natural hybridization between species provides an opportunity to study the mechanisms that maintain independent lineages and may help us understand the process of speciation. The New Zealand tree wētā speciesHemideinathoracicaproduces F1hybrids where it lives in sympatry with two closely related species:HemideinacrassidensandHemideinatrewicki. This study looked at the viability and fertility of F1hybrid wētā betweenH.thoracicaandH.crassidensthat were collected from the wild and kept in captivity. The hybrids appeared to have normal viability from the late juvenile stage, with all male wētā maturing at a late instar. Male F1hybrids displayed normal mating behavior and one male produced offspring in captivity. In contrast to Haldane’s rule, female F1hybrids appeared to be infertile; they refused to mate and did not produce eggs. No evidence ofWolbachiainfection was identified in any of the three North IslandHemideinaspecies.

Identification of novel genes significantly affecting growth in catfish through GWAS analysis.

Growth is the most important economic trait in aquaculture. Improvements in growth-related traits can enhance production, reduce costs and time to produce market-size fish. Catfish is the major aquaculture species in the United States, accounting for 65% of the US finfish production. However, the genes underlying growth traits in catfish were not well studied. Currently, the majority of the US catfish industry uses hybrid catfish derived from channel catfish female mated with blue catfish male. Interestingly, channel catfish and blue catfish exhibit differences in growth-related traits, and therefore the backcross progenies provide an efficient system for QTL analysis. In this study, we conducted a genome-wide association study for catfish body weight using the 250K SNP array with 556 backcross progenies generated from backcross of male F1 hybrid (female channel catfish×male blue catfish) with female channel catfish. A genomic region of approximately 1Mb on linkage group 5 was found to be significantly associated with body weight. In addition, four suggestively associated QTL regions were identified on linkage groups 1, 2, 23 and 24. Most candidate genes in the associated regions are known to be involved in muscle growth and bone development, some of which were reported to be associated with obesity in humans and pigs, suggesting that the functions of these genes may be evolutionarily conserved in controlling growth. Additional fine mapping or functional studies should allow identification of the causal genes for fast growth in catfish, and elucidation of molecular mechanisms of regulation of growth in fish.

Components of reproductive isolation between the closely related grasshopper species Chorthippus biguttulus and C. mollis

The formation and maintenance of species in nature is accomplished by the evolution of reproductive isolating mechanisms. To understand patterns of speciation and coexistence between species, it is crucial to obtain a complete synopsis of reproductive isolating barriers. We identified multiple reproductive isolation barriers between two closely related species, Chorthippus biguttulus and Chorthippus mollis, and quantified their respective contributions to isolation. To this end, we produced hybrids and backcrosses in the laboratory to examine reproductive isolation at multiple stages in the life history. The prezygotic barriers, based on chemical and acoustic cues, are currently the strongest impediments to gene flow between C. biguttulus and C. mollis. This indicates an important role for sexual selection in the maintenance of species isolation. However, extrinsic and intrinsic postzygotic isolation barriers also contributed to reproductive isolation. The virtual sterility of F1 hybrid males was based on both the dysfunctionality of their testes and intermediate behavioral mating traits (behavioral sterility). This study demonstrates that a cascade of reproductive isolating mechanisms reduces the gene flow between C. biguttulus and C. mollis. Our results further demonstrate that the courtship display in these species consists of multimodal signals and indicates a key role of chemical cues in reproductive isolation and speciation in grasshoppers. The identification of species isolating mechanisms is essential for understanding the processes that drive speciation. A profound understanding of reproductive isolating mechanisms and their fitness consequences in a system is required to predict a specific speciation scenario. Members of the grasshopper subfamily Gomphocerinae show an enormous diversity and complexity in courtship behavior, especially in acoustic displays. Here, we demonstrate that non-acoustic and postzygotic isolation mechanisms also strongly contribute to reproductive isolation between the closely related grasshopper species, C. biguttulus and C. mollis. In addition, our results suggest that ongoing gene flow between this species pair is unlikely and that sexual selection plays an important role in the maintenance of species isolation.

Xenogenesis‐Production of Channel Catfish × Blue Catfish Hybrid Progeny by Fertilization of Channel Catfish Eggs with Sperm from Triploid Channel Catfish Males with Transplanted Blue Catfish Germ Cells

AbstractPutative spermatogonia A from a fresh‐cell isolate or a density‐gradient‐centrifuged isolate from the testes of Blue Catfish Ictalurus furcatus were transplanted into the gonads of triploid Channel Catfish I. punctatus. The cells were introduced into gonads of the host via catheterization (2 × 104–1.43 × 106 cells) or by surgically inserting the cells directly into the gonad (7 × 104–1.25 × 105 cells). Ten months after implantation, DNA was analyzed from biopsies of the gonads and seven of eight males were found to be xenogenic, having Blue Catfish cells in their gonads. The xenogenic males successfully courted normal Channel Catfish that had been induced with hormones to ovulate, but none of the eggs hatched, indicating inadequate sperm production, an inability to ejaculate, and/or low sperm quality. Male xenogenic catfish treated with luteinizing hormone releasing hormone analog had well‐developed testes, and sperm production was detected in three of seven xenogenic males examined 2 years after transplantation. Sperm were removed from a male that had been surgically transplanted with Blue Catfish cells and used to fertilize eggs from a hand‐stripped Channel Catfish female. One percent of these eggs hatched. All seven surviving 6‐month‐old progeny of this male had the external morphology, swim bladder shape, nuclear DNA profile, and mitochondrial DNA profile of female Channel Catfish × male Blue Catfish F1 hybrids, indicating that this triploid Channel Catfish male produced Blue Catfish sperm. This is the first report of successful production of xenogenic catfish and the first report of producing 100% hybrid progeny using xenogenesis in fish.

Spontaneous polyploidy, gynogenesis and androgenesis in second generation (F2 ) koi Cyprinus carpio × goldfish Carassius auratus hybrids.

The objective of this study was to characterize the genetics of second generation (F2 ) koi Cyprinus carpio × goldfish Carassius auratus hybrids. Spermatozoa produced by a novel, fertile F1 male were found to be diploid by flow-cytometric analysis. Backcross (F1 female × C. carpio male and C. carpio female × F1 male) juveniles were triploid, confirming that female and male F1 hybrids both produced diploid gametes. The vast majority of surviving F2 juveniles was diploid and small proportions were aneuploid (2·1n-2·3n and 3·1n-3·9n), triploid (3n) and tetraploid (4n). Microsatellite genotyping showed that F2 diploids repeated either the complete maternal or the complete paternal genotype. Fish with the maternal genotype were female and fish with the paternal genotype were male. This demonstrates that F2 diploids were the result of spontaneous gynogenesis and spontaneous androgenesis. Analysis of microsatellite inheritance and the sex ratio in F2 crosses showed that spontaneous gynogenesis and androgenesis did not always occur in equal proportions. One cross was found to have an approximate equal number of androgenetic and gynogenetic offspring while in several other crosses spontaneous androgenesis was found to occur more frequently than spontaneous gynogenesis.

Hybrid Dysfunction Expressed as Elevated Metabolic Rate in Male Ficedula Flycatchers.

Studies of ecological speciation are often biased towards extrinsic sources of selection against hybrids, resulting from intermediate hybrid morphology, but the knowledge of how genetic incompatibilities accumulate over time under natural conditions is limited. Here we focus on a physiological trait, metabolic rate, which is central to life history strategies and thermoregulation but is also likely to be sensitive to mismatched mitonuclear interactions. We measured the resting metabolic rate of male collared, and pied flycatchers as well as of naturally occurring F1 hybrid males, in a recent hybrid zone. We found that hybrid males had a higher rather than intermediate metabolic rate, which is indicative of hybrid physiological dysfunction. Fitness costs associated with elevated metabolic rate are typically environmentally dependent and exaggerated under harsh conditions. By focusing on male hybrid dysfunction in an eco-physiological trait, our results contribute to the general understanding of how combined extrinsic and intrinsic sources of hybrid dysfunction build up under natural conditions.

A genome-wide association study of heat stress-associated SNPs in catfish.

Heat tolerance is a complex and economically important trait for catfish genetic breeding programs. With global climate change, it is becoming an increasingly important trait. To better understand the molecular basis of heat stress, a genome-wide association study (GWAS) was carried out using the 250K catfish SNP array with interspecific backcross progenies, which derived from crossing female channel catfish with male F1 hybrid catfish (female channel catfish×male blue catfish). Three significant associated SNPs were detected by performing an EMMAX approach for GWAS. The SNP located on linkage group 14 explained 12.1% of phenotypical variation. The other two SNPs, located on linkage group 16, explained 11.3 and 11.5% of phenotypical variation respectively. A total of 14 genes with heat stress related functions were detected within the significant associated regions. Among them, five genes-TRAF2, FBXW5, ANAPC2, UBR1 and KLHL29- have known functions in the protein degradation process through the ubiquitination pathway. Other genes related to heat stress include genes involved in protein biosynthesis (PRPF4 and SYNCRIP), protein folding (DNAJC25), molecule and iron transport (SLC25A46 and CLIC5), cytoskeletal reorganization (COL12A1) and energy metabolism (COX7A2, PLCB1 and PLCB4) processes. The results provide fundamental information about genes and pathways that is useful for further investigation into the molecular mechanisms of heat stress. The associated SNPs could be promising candidates for selecting heat-tolerant catfish lines after validating their effects on larger and various catfish populations.

Postmating Reproductive isolation between strains of Drosophila willistoni

ABSTRACTSpeciation can occur through the presence of reproductive isolation barriers that impede mating, restrict cross-fertilization, or render inviable/sterile hybrid progeny. The D. willistoni subgroup is ideally suited for studies of speciation, with examples of both allopatry and sympatry, a range of isolation barriers, and the availability of one species complete genome sequence to facilitate genetic studies of divergence. D. w. willistoni has the largest geographic distribution among members of the Drosophila willistoni subgroup, spanning from Argentina to the southern United States, including the Caribbean islands. A subspecies of D. w. willistoni, D. w. quechua, is geographically separated by the Andes mountain range and has evolved unidirectional sterility, in that only male offspring of D. w. quechua females × D. w. willistoni males are sterile. Whether D. w. willistoni flies residing east of the Andes belong to one or more D. willistoni subspecies remains unresolved. Here we perform fecundity assays and show that F1 hybrid males produced from crosses between different strains found in Central America, North America, and northern Caribbean islands are reproductively isolated from South American and southern Caribbean island strains as a result of unidirectional hybrid male sterility. Our results show the existence of a reproductive isolation barrier between the northern and southern strains and suggest a subdivision of the previously identified D. willistoni willistoni species into 2 new subspecies.

Genetic Links between Recombination and Speciation

Genetic Links between Recombination and Speciation

Variation in the nasal cavity of baboon hybrids with implications for late Pleistocene hominins

Hybridization is increasingly proving to be an important force shaping human evolution. Comparisons of both ancient and modern genomes have provided support for a complex evolutionary scenario over the past million years, with evidence for multiple incidents of gene exchange. However, to date, genetic evidence is still limited in its ability to pinpoint the precise time and place of ancient admixture. For that we must rely on evidence of admixture from the skeleton. The research presented here builds on previous work on the crania of baboon hybrids, focusing on the nasal cavity of olive baboons, yellow baboons, and first generation (F1) hybrids. The nasal cavity is a particularly important anatomical region for study, given the clear differentiation of this feature in Neanderthals relative to their contemporaries, and therefore it is a feature that will likely differ in a distinctive manner in hybrids of these taxa. Metric data consist of 45 linear, area, and volume measurements taken from CT scans of known-pedigree baboon crania. Results indicate that there is clear evidence for differences among the nasal cavities of the parental taxa and their F1 hybrids, including a greater degree of sexual dimorphism in the hybrids. There is also some evidence for transgressive phenotypes in individual F1 animals. The greatest amount of shape variation occurs in the anterior bony cavity, the choana, and the mid-nasopharynx. Extrapolating our results to the fossil record, we would expect F1 hybrid fossils to have larger nasal cavities, on average, than either parental taxon, with overall nasal cavity shape showing the most profound changes in regions that are distinct between the parental taxa (e.g., anterior nasal cavity). We also expect size and shape differences to be more pronounced in male F1 hybrids than in females. Because of pronounced anterior nasal cavity differences between Neanderthals and their contemporaries, we suggest that this model might be effective for examining the fossil record of late Pleistocene contact.

Feed intake and growth performance of growing pigs fed on Acacia tortilis leaf meal treated with polyethylene glycol

The objective of the study was to determine the response in feed intake and performance of pigs fed on incremental levels of polyethylene glycol (PEG). Forty-eighty male F1 hybrid pigs were randomly allotted to six diets containing 0, 5, 10, 15, 20, and 25 g/kg of PEG, respectively. Acacia tortilis leaf meal was included at a rate of 150 g/kg. Each diet was offered ad libitum to eight pigs in individual pens. Average daily feed intake (ADFI), scaled feed intake (SFI), average daily gain (ADG), and gain to feed ratio (G:F) were determined weekly. The ADG showed a linear response to PEG (p < 0.01). The linear regression equation was y = 0.0061x + 0.6052 (R(2) = 0.64). There was a quadratic response to PEG on ADFI, and SFI (p < 0.01) and G:F (p > 0.05). The regression equations and R(2) values were as follows: ADFI y = 0.0008x(2) - 00086x + 1.2339 (R(2) = 0.96), SFI y = 0.0147x(2) - 0.2349x + 40.096 (R(2) = 0.95), and G:F ratio y = 0.0002x(2) - 0.0017x + 0.5168 (R(2) = 0.56). The ADFI, SFI, and ADG increased as weeks of feeding progressed (p < 0.01), but the G:F ratio decreased as weeks increased. It can be concluded that the relationship between PEG inclusion and performance of growing pigs fed on A. tortilis is exponential, rather than linear. The economic benefit of using PEG depend on cost of labor, availability of Acacia, costs of harvesting together with processing, and acceptability of the pork.

Common Tern Sterna hirundo and Arctic Tern S. paradisaea hybridization produces fertile offspring

This study provides the first molecular evidence for hybridization between Arctic Terns Sterna paradisaea and Common Terns S. hirundo. We studied a mixed pair (P1; male Arctic Tern and female Common Tern) and its offspring on Penikese Island, Massachusetts, USA in 2007–2015. The pair maintained a long-term pair bond (8 years); its reproductive performance was comparable to that of Common Terns and higher than that of Arctic Terns at the site. Molecular analyses confirmed that all young raised by the pair (at least 5 males and 4 females) were biological offspring. We describe hybrid young and the adult hybrid in detail to facilitate field identification. Although F1 hybrid young were intermediate between the parent species in certain characteristics, dark feathers extending below the eye, in particular, gave them a distinctly Arctic Tern-like appearance; however, all had secondary feathers that were darker than the wing-coverts, like Common Terns. We detected one male F1 hybrid that returned to breed; it retained some features intermediate between the parent species, but we qualitatively judged it to be more Common Tern-like. It mated with a Common Tern and produced three confirmed backcross hybrid young that closely resembled Common Terns. We speculate that rarity of Arctic Terns, especially females, at and near the study site and age/inexperience of the Common Tern parent were proximate mechanisms for the formation of the P1 pair. Further, because mixed pairs are occasionally observed and hybrids are cryptic, we suggest that Arctic/Common Tern hybridization may occur more commonly than is currently realized.

Heterosis Increases Fertility, Fecundity, and Survival of Laboratory-Produced F1 Hybrid Males of the Malaria Mosquito Anopheles coluzzii.

The success of vector control strategies aiming to decrease disease transmission via the release of sterile or genetically-modified male mosquitoes critically depends on mating between laboratory-reared males and wild females. Unfortunately, mosquito colonization, laboratory rearing, and genetic manipulations can all negatively affect male competitiveness. Heterosis is commonly used to produce domestic animals with enhanced vigor and homogenous genetic background and could therefore potentially improve the mating performance of mass-reared male mosquitoes. Here, we produced enhanced hybrid males of the malaria mosquito Anopheles coluzzii by crossing two strains colonized >35 and 8 years ago. We compared the amount of sperm and mating plug proteins they transferred to females, as well as their insemination rate, reproductive success and longevity under various experimental conditions. Across experiments, widespread adaptations to laboratory mating were detected in the older strain. In large-group mating experiments, no overall hybrid advantage in insemination rates and the amount of sperm and accessory gland proteins transferred to females was detected. Despite higher sperm activity, hybrid males did not appear more fecund. However, individual-male mating and laboratory-swarm experiments revealed that hybrid males, while inseminating fewer females than older inbred males, were significantly more fertile, producing larger mating plugs and drastically increasing female fecundity. Heterotic males also showed increased longevity. These results validate the use of heterosis for creating hybrid males with improved fitness from long-established inbred laboratory strains. Therefore, this simple approach could facilitate disease control strategies based on male mosquito releases with important ultimate benefits to human health.

A genome-wide association study in catfish reveals the presence of functional hubs of related genes within QTLs for columnaris disease resistance.

BackgroundColumnaris causes severe mortalities among many different wild and cultured freshwater fish species, but understanding of host resistance is lacking. Catfish, the primary aquaculture species in the United States, serves as a great model for the analysis of host resistance against columnaris disease. Channel catfish in general is highly resistant to the disease while blue catfish is highly susceptible. F2 generation of hybrids can be produced where phenotypes and genotypes are segregating, providing a useful system for QTL analysis. To identify genes associated with columnaris resistance, we performed a genome-wide association study (GWAS) using the catfish 250 K SNP array with 340 backcross progenies derived from crossing female channel catfish (Ictalurus punctatus) with male F1 hybrid catfish (female channel catfish I. punctatus × male blue catfish I. furcatus).ResultsA genomic region on linkage group 7 was found to be significantly associated with columnaris resistance. Within this region, five have known functions in immunity, including pik3r3b, cyld-like, adcyap1r1, adcyap1r1-like, and mast2. In addition, 3 additional suggestively associated QTL regions were identified on linkage groups 7, 12, and 14. The resistant genotypes on the QTLs of linkage groups 7 and 12 were found to be homozygous with both alleles being derived from channel catfish. The paralogs of the candidate genes in the suggestively associated QTL of linkage group 12 were found on the QTLs of linkage group 7. Many candidate genes on the four associated regions are involved in PI3K pathway that is known to be required by many bacteria for efficient entry into the host.ConclusionThe GWAS revealed four QTLs associated with columnaris resistance in catfish. Strikingly, the candidate genes may be arranged as functional hubs; the candidate genes within the associated QTLs on linkage groups 7 and 12 are not only co-localized, but also functionally related, with many of them being involved in the PI3K signal transduction pathway, suggesting its importance for columnaris resistance.

The effects of extremely low frequency magnetic fields on mutation induction in mice

The growing human exposure to extremely low frequency (ELF) magnetic fields has raised a considerable concern regarding their genotoxic effects. The aim of this study was to evaluate the in vivo effects of ELF magnetic fields irradiation on mutation induction in the germline and somatic tissues of male mice. Seven week old BALB/c×CBA/Ca F1 hybrid males were exposed to 10, 100 or 300μT of 50Hz magnetic fields for 2 or 15h. Using single-molecule PCR, the frequency of mutation at the mouse Expanded Simple Tandem Repeat (ESTR) locus Ms6-hm was established in sperm and blood samples of exposed and matched sham-treated males. ESTR mutation frequency was also established in sperm and blood samples taken from male mice exposed to 1Gy of acute X-rays. The frequency of ESTR mutation in DNA samples extracted from blood of mice exposed to magnetic fields did not significantly differ from that in sham-treated controls. However, there was a marginally significant increase in mutation frequency in sperm but this was not dose-dependent. In contrast, acute exposure X-rays led to significant increases in mutation frequency in sperm and blood of exposed males. The results of our study suggest that, within the range of doses analyzed here, the in vivo mutagenic effects of ELF magnetic fields are likely to be minor if not negligible.

A male pheromone-mediated trade-off between female preferences for genetic compatibility and sexual attractiveness in rats

IntroductionChemosensory signals play a vital role in socio-sexual interactions of rodents. Females rely heavily on chemosensory signals to evaluate genetic similarity and quality of potential mates, but their olfactory preferences for these criteria often conflict in mate choice.ResultsUsing two inbred strains of rats, Brown Norway (BB) and Lewis (LL) and their F1 reciprocal hybrids (BL, BB♀ breed with LL♂; LB, LL♀ breed with BB♀) as genetic models, we found that the chemosensory preferences of BB and LL females between these 4 strains of rats could be predicted on the basis of genetic compatibility benefits, except that LL females exhibited incestuous preferences for male urine odor of LL rats over that of the BB strain and the F1 hybrids. Seven ketone components of major urine volatiles proved to be potential male pheromones and were enriched in LL males compared to BB males or the F1 hybrid males. We hypothesize that these ketones produced an extravagant male trait that attracts LL females, overriding compatibility traits. This conclusion was corroborated by adding three synthetic pheromone analogues, 4-heptanone, 2-heptanone and 9-hydroxy-2-nonanone of these 7 components, which resulted in equalization of the sexual attractiveness of BB male urine and LL male urine. Additionally, in the genetically diverse F2 hybrids (BL♀ breed with BL♂), the pheromones-enriched males could consistently attract the F2 females.ConclusionsWe suggest that the exaggerated male pheromones serve as a “sexual chemical ornament” to attract females, independent of genetic compatibility, whereas genetic dissimilarity could influence the preferences only when male pheromones varied on a small scale.

Gonad morphogenesis defects drive hybrid male sterility in asymmetric hybrid breakdown of Caenorhabditis nematodes.

Determining the causes and evolution of reproductive barriers to gene flow between populations, speciation, is the key to understanding the origin of diversity in nature. Many species manifest hybrid breakdown when they intercross, characterized by increasingly exacerbated problems in later generations of hybrids. Recently, Caenorhabditis nematodes have emerged as a genetic model for studying speciation, and here we investigate the nature and causes of hybrid breakdown between Caenorhabditis remanei and C. latens. We quantify partial F1 hybrid inviability and extensive F2 hybrid inviability; the ~75% F2 embryonic arrest occurs primarily during gastrulation or embryonic elongation. Moreover, F1 hybrid males exhibit Haldane's rule asymmetrically for both sterility and inviability, being strongest when C. remanei serves as maternal parent. We show that the mechanism by which sterile hybrid males are incapable of transferring sperm or a copulatory plug involves defective gonad morphogenesis, which we hypothesize results from linker cell defects in migration and/or cell death during development. This first documented case of partial hybrid male sterility in Caenorhabditis follows expectations of Darwin's corollary to Haldane's rule for asymmetric male fitness, providing a powerful foundation for molecular dissection of intrinsic reproductive barriers and divergence of genetic pathways controlling organ morphogenesis.

Paternal inheritance in mealybugs (Hemiptera: Coccoidea: Pseudococcidae).

Mealybugs have a haplodiploid reproduction system, with paternal genome elimination (PGE); the males are diploid soon after fertilization, but during embryogenesis, the male paternal set of chromosomes becomes heterochromatic (HC) and therefore inactive. Previous studies have suggested that paternal genes can be passed on from mealybug males to their sons, but not necessarily by any son, to the next generation. We employed crosses between two mealybug species--Planococcus ficus (Signoret) and Planococcus citri (Risso)--and between two populations of P. ficus, which differ in their mode of pheromone attraction, in order to demonstrate paternal inheritance from males to F2 through F1 male hybrids. Two traits were monitored through three generations: mode of male pheromone attraction (pherotype) and sequences of the internal transcribed spacer 2 (ITS2) gene segment (genotype). Our results demonstrate that paternal inheritance in mealybugs can occur from males to their F2 offspring, through F1 males (paternal line). F2 backcrossed hybrid males expressed paternal pherotypes and ITS2 genotypes although their mother originated through a maternal population. Further results revealed other, hitherto unknown, aspects of inheritance in mealybugs, such as that hybridization between the two species caused absence of paternal traits in F2 hybrid females produced by F1 hybrid females. Furthermore, hybridization between the two species raised the question of whether unattracted males have any role in the interactions between P. ficus and P. citri.

Reduced recombination patterns in Robertsonian hybrids between chromosomal races of the house mouse: chiasma analyses.

The recombination suppression models of chromosomal speciation posit that chromosomal rearrangements act as partial barriers to gene flow allowing these regions to accumulate genetic incompatibilities, thus contributing to the divergence of populations. Empirical and theoretical studies exploring the requirements of these models have mostly focused on the role of inversions. Here, the recombination landscape of heterozygosity for Robertsonian (Rb) fusions is investigated in the house mouse. Laboratory-bred F1 males and females between highly differentiated races from Tunisia (Rb: 2n=22, Standard, St: 2n=40) were produced in which all Rb fusions are present as trivalents in meiosis. Recombination patterns were determined by the analysis of chiasmata and compared with previous data on the Tunisian parental mice. A comparative analysis was performed on wild-caught male mice spanning the hybrid zone between two Italian races (2n=40, 2n=22). The results showed that the chiasma characteristics of both male and female Tunisian F1 and Italian hybrids clearly differed from those of Rb and St mice. Not only was the mean chiasma number (CN) intermediate between those of the parental mice in both geographic samples, but the distribution of chiasmata along the chromosomal arms of the F1 showed a distinct mosaic pattern. In short, the proximal region in the F1 exhibited a reduced CN similar to that observed in homozygous Rb, whereas distal regions more closely matched those in St mice. These results suggest that Rb rearrangements (homozygous or heterozygous) reduce recombination in the proximal regions of the chromosomes supporting their potential role in recombination-mediated speciation models.