Genetic regulation of the replication pattern of polytene chromosomes in interspecific hybrids of Drosophila

The sixth chromosome (microchromosome) of D. littoralis changed its replication pattern in nuclei of the salivary gland cells in reciprocal F₁ hybrids between D. virilis and D. littoralis. — To locate the factor (or factors) in the D. virilis chromosomes, which may influence the replication pattern of the sixth chromsome of D. littoralis, we produced hybrid stocks with synthetic karyotypes characterized by different combinations of D. littoralis homologous chromosomes and hybrid chromosomes. Based on autoradiographical studies of DNA synthesis in synthetic karyotypes, it may be concluded that the dominant factor (or factors), which influences the replication of the sixth chromosome of D. littoralis, is located on the homeologous microchromosomes of D. virilis. The possible interrelation between the changed replication pattern of D. littoralis sixth chromosome and its atypical behaviour at early embryogenesis in (D. virilis x D. littoralis) F₁ hybrids is discussed.     

Genetic regulation of CFA expression in interspecific avian hybrids and somatic cell hybrids

Chicken fetal-leukemic antigen (CFA) is an oncodevelopmental antigen present on embryonic and neonatal chicken peripheral red blood cells (RBCs) but is not restricted to fetal stages of development in other avian species. Crosses between white Leghorn chickens and Japanese quail resulted in adult hybrids whose peripheral RBCs were positive for CFA. Of the four CFA determinants normally found in adult quail RBCs, only two were present on quail-chicken hybrid RBCs. Adult quail--chicken hybrid RBCs also possessed on CFA determinant associated with early development in both quail and chicken and one chicken-specific CFA determinant. Evidence is presented for the possible association of CFA-positive adult peripheral RBCs and the level of circulating reticulocytes. Crosses between pheasant and turkey (both with CFA-positive adult RBCs) resulted in hybrid adult RBCs expressing only a portion of the parental CFA determinants. Through the formation of somatic cell hybrids between adult chicken and embryonic Japanese quail RBCs, it was possible to induce the appearance of CFA determinants normally restricted to embryonic chicken RBCs. Approximately 50% of the hybrid cells showed reexpression of CFA, and this induction was both time and temperature dependent. Hybridization between RBCs of adult chicken and those of either adult Japanese quail or adult turkey failed to elicit the reexpression of chicken-specific CFA.     

The pattern of polytene chromosome synapsis in Drosophila species and interspecific hybrids

The pairing of polytene chromosomes was investigated in Drosophila melanogaster, Drosophila simulans and their hybrids as well as in species of the D. virilis group and in F₁ hybrids between the species of this group. The study of frequency and extent of asynapsis revealed non-random distribution along chromosome arms both in interspecific hybrids and pure Drosophila species. It is suggested that definite chromosome regions exhibiting high pairing frequency serve as initiation sites of synapsis in salivary gland chromosomes.     

The pattern of polytene chromosome conjugation and crossing-over in interspecific hybrids of Drosophila

Summary The pairing of polytene chromosomes was investigated in the hybrids between three closely related species of Drosophila belonging to the virilis species group. It was found that within the same hybrid different chromosome bands lost the ability to pair by differing degrees. Furthermore, the same chromosome sections paired with different frequencies depending on the hybrid involved. This study revealed that poor polytene chromosome pairing in the hybrids is not due to specific genetic interaction in the hybrids, but depends solely on the properties of the homologous loci themselves. It was also of interest to find whether the pattern of polytene chromosome somatic pairing resembled in some way the picture of chromosome synapsis during meiosis. To obtain evidence for this, crossing-over in the hybrid 5th chromosome was analyzed both genetically and cytologically (from salivary gland chromosome observations). It was found that the sections of the fifth chromosome which were characterized by a high frequency of conjugation in the salivary glands of hybrids also exhibited a high frequency of crossing-over in hybrid females. It may be concluded that sections of the polytene chromosome characterized by a low frequency of conjugation behave in the same manner in meiosis, and thus rarely take part in genetic recombination.     

Transposition of mobile genetic elements in interspecific hybrids of Drosophila

In situ hybridization of labeled DNA of four mobile dispersed genetic elements (mdg), isolated from D. melanogaster and C. virilis genomes, with polytene chromosomes of the larvae of several Drosophila species has been carried out. The data show that the mdg elements exhibit a high degree of species specificity. The same conclusions are derived from filter hybridization using ³²P-labeled D. melanogaster and D. virilis DNA and cloned mdg sequences immobilized on nitrocellulose filters. We attempted to induce transpositions (jumping) of mdg elements specific for D. virilis chromosomes to the chromosomes of related species (e.g. D. littoralis Meigen) originally lacking the representatives of this family of repeats. For this purpose we produced hybrid stocks with synthetic karyotoypes characterized by different combinations of D. virilis homologous chromosomes and hybrid chromosomes. In one of such stocks we did find by in situ hybridization the insertion of a D. virilis mdg element into the fifth chromosome of D. littoralis Meigen. The transposition (jumping) took place in the only region where somatic pairing between the fifth chromosomes of D. virilis and D. littoralis occurs more or less regularly in the hybrids. Since crossing-over in hybrid chromosomes of males is excluded in such synthetic stocks, gene conversion may be responsible for this transposition. The possible bearing of the phenomenon observed on the problem of hybrid dysgenesis is discussed.     

Host-related life history traits in interspecific hybrids of cactophilic Drosophila

In the genus Drosophila (Diptera: Drosophilidae), interspecific hybridization is a rare phenomenon. However, recent evidence suggests a certain degree of introgression between the cactophilic siblings Drosophila buzzatii Patterson & Wheeler and Drosophila koepferae Fontdevila & Wasserman. In this article, we analyzed larval viability and developmental time of hybrids between males of D. buzzatii and females of D. koepferae, raised in media prepared with fermenting tissues of natural host plants that these species utilize in nature as breeding sites. In all cases, developmental time and larval viability in hybrids was not significantly different from parental lines and, depending on the cross, hybrids developed faster than both parental species or than the slowest species. When data of wing length were included in a discriminant function analysis, we observed that both species can be clearly differentiated, while hybrids fell in two categories, one intermediate between parental species and the other consisting of extreme phenotypes. Thus, our results point out that hybrid fitness, as measured by developmental time and viability, is not lower than in the parental species.     

Tryptophan operon regulation in interspecific hybrids of enteric bacteria.

We examined tryptophan regulation in merodiploid hybrids in which a plasmid carrying the trp operon of Escherichia was introduced into Trp mutants of other enteric genera, or in which a plasmid carrying the trpR+ (repressor) gene of E. coli was transfered into fully constitutive trpR mutants of other genera. In these hybrids the trp operon of one species is controlled by the repressor of a different species. Similar investigations were possible in transduction hybrids in which either the trp operon or the trpR+ locus of Shigella dysenteriae was introduced into E. coli. Our measurements of trp enzymes levels in repressed and nonrepressed cells indicate that Trp regulation is normal, with only minor quantitative variations, in hybrids between E coli and Shigella dysenteriae, Salmonella typhimurium, Klebsiella aerogenes, Serratia marcescens, and Proteus mirabilis. Our results support the idea that a repressor-operator mechanism for regulating trp messenger ribonucleic acid production evolved in a common ancestor of the enteric bacteria, and that this repressor-operator recognition has been conversed during the evolutionary divergence of the Enterobacteriaceae.     

Genetic control of tumorigenicity in interspecific mammalian cell hybrids.

The nature of genetic control of cellular malignancy was investigated by examining the tumorigenicity of a series of interspecific mouse-human cell hybrids in the athymic nude mouse. Two highly malignant but genetically distinct mouse cell lines, A9 and PG19, were hybridized with three normal human diploid fibroblast strains, and 19 independently arising hybrid clones were isolated. Each of these clones was capable of forming progressive lethal tumors in the nude mouse, and thus resembled the malignant parental mouse cells rather than the nonmalignant parental human cells. We failed to obtain any evidence for complete suppression of tumorigenicity in these cell hybrids. The absence of suppression was observed regardless of the extent and composition of the human chromosome complements retained in the hybrid clones; the results of detailed cytological and isoenzyme analyses would make it highly improbable that the observed lack of suppression was due to cellular selection in vivo for a more tumorigenic subpopulation in the injected hybrid cells. These data demonstrate that at least for the parental cell combinations used in this study, no human chromosome, when present singly in the mouse-human cell hybrids, can suppress the tumorigenic phenotype of the mouse cells. Our results are consistent with the view that the suppression of cellular malignancy previously demonstrated in intraspecific (mouse × mouse) somatic cell hybrids does not occur in interspecific (mouse-human) cell hybrids, or alternatively, genetic determinants located on two or more human chromosomes are required simultaneously to suppress the malignancy of the mouse cells in cell hybrids derived from malignant mouse cell and nonmalignant human cells.     

Symmetry breaking in interspecific Drosophila hybrids is not due to developmental noise

Hybrids from crosses of different species have been reported to display decreased developmental stability when compared to their pure species, which is conventionally attributed to a breakdown of coadapted gene complexes. Drosophila subobscura and its close relative D. madeirensis were hybridized in the laboratory to test the hypothesis that genuine fluctuating asymmetry, measured as the within-individual variance between right and left wings that results from random perturbations in development, would significantly increase after interspecific hybridization. When sires of D. subobscura were mated to heterospecific females following a hybrid half-sib breeding design, F1 hybrid females showed a large bilateral asymmetry with a substantial proportion of individuals having an asymmetric index larger than 5% of total wing size. Such an anomaly, however, cannot be plainly explained by an increase of developmental instability in hybrids but is the result of some aberrant developmental processes. Our findings suggest that interspecific hybrids are as able as their parents to buffer developmental noise, notwithstanding the fact that their proper bilateral development can be harshly compromised. Together with the low correspondence between the co-variation structures of the interindividual genetic components and the within-individual ones from a Procrustes analysis, our data also suggest that the underlying processes that control (genetic) canalization and developmental stability do not share a common mechanism. We argue that the conventional account of decreased developmental stability in interspecific hybrids needs to be reappraised.     

Genetic recombination in Sorghum bicolor x S. macrospermum interspecific hybrids

Sorghum has been improved by public and private breeding programs utilizing germplasm mostly from within the species Sorghum bicolor. Until recently, cross-incompatibilities have prevented hybridization of S. bicolor with most other species within the genus Sorghum. Utilizing germplasm homozygous for the iap allele, hybrids were readily produced between S. bicolor (2n = 20; AAB1B1) and S. macrospermum (2n = 40; WWXXYYZZ). These hybrids were intermediate to the parents in chromosome number (2n = 30) and overall morphology. Meiosis in both parents was regular; S. bicolor had 10 bivalents per pollen mother cell (PMC) and S. macrospermum had an average of 19.96 bivalents per PMC. Six hybrids were studied cytologically and meiosis was irregular, with the chromosomes associating primarily as univalents and bivalents. There was an average of 3.54 bivalents per PMC, with a range of 0-8 bivalents, most of which were rods (98%). Using FISH (fluorescent in situ hybridization), moderate levels (2.6 II per PMC) of allosyndetic recombination were observed. Genomic relationships were sufficient to assign S. macrospermum the genomic formula AAB1B1YYZZ (Y and Z remain unknown). Allosyndetic recombination in the interspecific hybrids indicates that introgression through genetic recombination should be possible if viable backcrosses can be recovered.     

Genetic screens for factors involved in the notum bristle loss of interspecific hybrids between Drosophila melanogaster and D. simulans

Interspecific cross is a powerful means to uncover hidden within- and between-species variation in populations. One example is a bristle loss phenotype of hybrids between Drosophila melanogaster and D. simulans, although both the pure species have exactly the same pattern of bristle formation on the notum. There exists a large amount of genetic variability in the simulans populations with respect to the number of missing bristles in hybrids, and the variation is largely attributable to simulans X chromosomes. Using nine molecular markers, I screened the simulans X chromosome for genetic factors that were responsible for the differences between a pair of simulans lines with high (H) and low (L) missing bristle numbers. Together with duplication-rescue experiments, a single major quantitative locus was mapped to a 13F-14F region. Importantly, this region accounted for most of the differences between H and L lines in three other independent pairs, suggesting segregation of H and L alleles at the single locus in different populations. Moreover, a deficiency screening uncovered several regions with factors that potentially cause the hybrid bristle loss due to epistatic interactions with the other factors.     

Inducing chromosome pairing through premature condensation: analysis of wheat interspecific hybrids

At the onset of meiosis, chromosomes first decondense and then condense as the process of recognition and intimate pairing occurs between homologous chromosomes. We show here that okadaic acid, a drug known to induce chromosome condensation, can be introduced into wheat interspecific hybrids prior to meiosis to induce chromosome pairing. This pairing occurs in the presence of the Ph1 locus, which usually suppresses pairing of related chromosomes and which we show here delays condensation. Thus the timing of chromosome condensation during the onset of meiosis is an important factor in controlling chromosome pairing.     

Compensatory cis-trans evolution and the dysregulation of gene expression in interspecific hybrids of Drosophila

Hybrids between species are often characterized by novel gene-expression patterns. A recent study on allele-specific gene expression in hybrids between species of Drosophila revealed cases in which cis- and trans-regulatory elements within species had coevolved in such a way that changes in cis-regulatory elements are compensated by changes in trans-regulatory elements. We hypothesized that such coevolution should often lead to gene misexpression in the hybrid. To test this hypothesis, we estimated allele-specific expression and overall expression levels for 31 genes in D. melanogaster, D. simulans, and their F1 hybrid. We found that 13 genes with cis-trans compensatory evolution are in fact misexpressed in the hybrid. These represent candidate genes whose dysregulation might be the consequence of coevolution of cis- and trans-regulatory elements within species. Using a mathematical model for the regulation of gene expression, we explored the conditions under which cis-trans compensatory evolution can lead to misexpression in interspecific hybrids.     

Temperature sensitivity of genes controlling chromosome elimination in Drosophila hybrids]

The thermosensitivity and thermosensitive period of the genes controlling the elimination of the 6th chromosome of D. littoralis in the hybrids male D. virillis X female D. littoralis were studied. The appearance of flies with the mutation glossy (mosaics and haplo-6-flies) served as a criterion of chromosome elimination. The genes under study were shown to be cold-sensitive, monophasic. The thermosensitive period lasts 2.5 hrs after egg laying--from the 1st cleavage division till the beginning of migration of the nuclei in the egg cortex. The appearance of almost 100% of haplo-6-flies at at 17 degrees is accounted for by the synchronous elimination of the 6th chromosome of D. littoralis during the first 3 cleavage divisions. The appearance of mosaics at 25 degrees is accounted for by the asynchronous chromosome elimination.     

Sex-specific asymmetry in eye development in interspecific hybrids in the Drosophila bipectinata species complex

Journal of Genetics -     

Genetic variation and relationships of seven sturgeon species and ten interspecific hybrids

Background

Sturgeon cultivation is important for both industry and aquaculture in China. To date, more than 17 species or strains have been farmed for fillets and caviar production. Crossbreeding among different sturgeon species is frequent and the F2 hybrids are fertile. However, large-scale farming can have negative impacts on wild populations i.e. escape of exotic sturgeons and must be taken into consideration. Escape of exotic sturgeons can cause severe ecological problems, including threatening native sturgeon species once the exotic varieties become established or hybridize with native individuals. However, little is known about their genetic resources and variation.

Methods

Genetic diversity and introgression of seven sturgeon species were analyzed using mitochondrial DNA cytochrome oxidase subunit I (COI) and nine microsatellite markers. This study included 189 individuals from seven sturgeon species and 277 individuals from ten lineages of F2 hybrid strains.

Results

MtDNA COI sequences (632 bp long) were generated from 91 individuals across the 17 sturgeon strains and produced 23 different haplotypes. Haplotype diversity was high (h = 0.915 ± 0.015) and nucleotide diversity was low (π = 0.03680 ± 0.00153) in the seven sturgeon species and ten interspecific hybrids. Phylogenetic analyses resulted in almost identical tree topologies, and different haplotype structures were mainly related with sturgeons of different female parents. Analysis of molecular variance revealed that 81.73% of the genetic variance was due to matrilineal differences, while 9.40% resulted from strain variation. Pairwise Fst values obtained with POLYSAT software, were high among strains and ranged from 0.031 to 0.164. Admixture analysis assigned seven distinct groups and ten genotypes of admixed clusters composed of hybrid strains using STRUCTURE when assuming K = 7.

Conclusions

The interspecific mtDNA gene tree corresponded to the expected taxonomic divisions. These relationships were also supported by the results from the microsatellite analysis and contributed to unambiguously identify seven sturgeon species and ten F2 hybrid strains from sturgeon farms in China. Moreover, we found that introgressive hybridization is pervasive, exists in both purebred and hybrid sturgeons, and may reflect widespread mismanagement in sturgeon breeding in China.     

Arginine vasotocin regulation of interspecific cooperative behaviour in a cleaner fish

In an interspecific cooperative context, individuals must be prepared to tolerate close interactive proximity to other species but also need to be able to respond to relevant social stimuli in the most appropriate manner. The neuropeptides vasopressin and oxytocin and their non-mammalian homologues have been implicated in the evolution of sociality and in the regulation of social behaviour across vertebrates. However, little is known about the underlying physiological mechanisms of interspecific cooperative interactions. In interspecific cleaning mutualisms, interactions functionally resemble most intraspecific social interactions. Here we provide the first empirical evidence that arginine vasotocin (AVT), a non-mammalian homologue of arginine vasopressin (AVP), plays a critical role as moderator of interspecific behaviour in the best studied and ubiquitous marine cleaning mutualism involving the Indo-Pacific bluestreak cleaner wrasse Labroides dimidiatus. Exogenous administration of AVT caused a substantial decrease of most interspecific cleaning activities, without similarly affecting the expression of conspecific directed behaviour, which suggests a differential effect of AVT on cleaning behaviour and not a general effect on social behaviour. Furthermore, the AVP-V1a receptor antagonist (manning compound) induced a higher likelihood for cleaners to engage in cleaning interactions and also to increase their levels of dishonesty towards clients. The present findings extend the knowledge of neuropeptide effects on social interactions beyond the study of their influence on conspecific social behaviour. Our evidence demonstrates that AVT pathways might play a pivotal role in the regulation of interspecific cooperative behaviour and conspecific social behaviour among stabilized pairs of cleaner fish. Moreover, our results suggest that the role of AVT as a neurochemical regulator of social behaviour may have been co-opted in the evolution of cooperative behaviour in an interspecific context, a hypothesis that is amenable to further testing on the potential direct central mechanism involved.