Enzyme patterns in two species of Xenopus and their hybrids

Differences in the isozyme patterns of Xenopus laevis and Xenopus mulleri have been utilized to examine the expression of alleles of both species in hybrid animals. Mitochondrial MDH and tetrazolium oxidase phenotypes were examined during the development of non-hybrid embryos of each species and of reciprocal hybrids. Early stages of the hybrids resemble the enzyme phenotype of the maternal parent. Appearance of paternal enzyme takes place just prior to the active feeding tadpole stage for both mitochondrial MDH and oxidase. The maternal effect disappears shortly thereafter in early feeding tadpoles, at which point reciprocal hybrids have identical isozyme patterns. There is no evidence for a predominance of one species over the other. Examination of feeding tadpoles and adult toads indicates that both laevis and mulleri expression is stable. The appearance of paternal mitochondrial MDH does not correspond to the time when other mitochondrial components begin to increase in Xenopus. Multiple bands of MDH in both species and of oxidase in laevis are probably not due to the aggregation of subunits produced by different alleles at the same locus. There is no evidence for the formation of “hybrid” molecules consisting of subunits of both species.     

Repression of nucleolar organizer activity in an interspecific hybrid of the genus Xenopus

Nucleolar expression in reciprocal hybrids between African frogs of the species Xenopus laevis and X. mulleri has been examined throughout ontogeny. Evidence is presented for the stagespecific regulation of nucleolar expression in these hybrids, and for a maternal effect that operates during the early development. Advantage has been taken of the nucleolar organizer deletion in X. laevis to demonstrate that the uninucleolate conditions of hybrid nuclei at certain developmental stages is the result of an allelic repression of the mulleri nucleolar organizer region by the laevis genome. Differences in nucleolar expression between the reciprocal hybrids and the parental species have been put on a quantitative basis for several selected tissues.     

Synchronous allelic expression at the glucosephosphate isomerase A and B loci in interspecific sunfish hybrids

Allelic isozymes of glucosephosphate isomerase at the Gpi-A and -B loci were separated by starch gel electrophoresis in the warmouth (Lepomis gulosus) and green sunfish (L. cyanellus). The specific tissue distributions and developmental expressions of the GPI-A₂, -AB, and -B₂ isozymes were not different between these two species. The synchrony of allelic expression in normal intraspecific sunfish crosses was demonstrated by means of an electrophoretic variant at the Gpi-B locus. In embryos formed from warmouth × green sunfish hybrid crosses, the paternal GPI-A₂ isozymes were first expressed at the same time in both reciprocal hybrids, at 21–25 hr after fertilization. The maternal and paternal GPI-B subunits were synchronously expressed in reciprocal hybrids just prior to hatching. The parental allelic isozymes at both loci showed codominant expression in all tissues of the mature F₁ hybrids. These results are consistent with the absence of allelic asynchrony and inhibition in interspecific hybrids formed from more evolutionarily related species.     

Mitochondrial ribosomal proteins in Xenopus laevis/X. mulleri interspecific hybrids.

The large subunits of mitochondrial ribosomes were isolated from two related frog species, Xenopus laevis and X. mulleri, and their proteins were compared by two-dimensional polyacrylamide gel electrophoresis. Three of the proteins observed in X. laevis are absent from X. mulleri, and four of the proteins observed in X. mulleri are absent from X. laevis. More than these seven such species-specific proteins may occur.Reciprocal crosses between frogs of the two species gave two groups of F1 hybrids. Nuclear genes in these hybrids derive equally from both species, while mitochondrial DNA (and therefore mitochondrial rRNA) derived exclusively from the maternal species. Electrophoretic analyses of the large subunit proteins of these F1 animals revealed that four of the species-specific proteins are present only when their corresponding species was the mother. While this result is consistent with the coding of these four proteins by mitochondrial DNA, it does not provide evidence against nuclear coding of these proteins. A fifth protein is absent from both F1 hybrids. A sixth is present in both F1 hybrids, and a seventh is present only when its corresponding species was the father. We conclude that at least these latter two mitochondrial ribosomal proteins are encoded by nuclear genes.     

Isozyme expression in F1 hybrids between carp and goldfish

Interspecific genetic differences in malate dehydrogenase (MDH), lactate dehydrogenase (LDH), superoxide dismutase (SOD), and esterase (EST) isozymes in carp (Cyprinus carpio) and goldfish (Carassius auratus) were used to examine the allelic expressions in the hybrid between these species. A unique liver SOD and muscle LDH phenotype unambiguously identifies all presumed hybrid individuals. There was no evidence of F₂ or backcross phenotypes in hybrid individuals. Liver MDH and EST phenotypes in hybrids show a preferential expression of goldfish isozymes. Variation in the levels of carp liver MDH isozymes may result from the polymorphism of a regulatory mutation affecting isozyme expression, leading to gene silencing after duplication.This work was supported through NSERC (Canada) grants to James P. Bogart and John F. Leatherland.     

Gene expression analysis of the ovary of hybrid females of <Emphasis Type="Italic">Xenopus laevis</Emphasis> and <Emphasis Type="Italic">X. muelleri</Emphasis>

Background  

Interspecific hybrids of frogs of the genus Xenopus result in sterile hybrid males and fertile hybrid females. Previous work has demonstrated a dramatic asymmetrical pattern of misexpression in hybrid males compared to the two parental species with relatively few genes misexpressed in comparisons of hybrids and the maternal species (X. laevis) and dramatically more genes misexpressed in hybrids compared to the paternal species (X. muelleri). In this work, we examine the gene expression pattern in hybrid females of X. laevis × X. muelleri to determine if this asymmetrical pattern of expression also occurs in hybrid females.     

Ontogeny of lactate dehydrogenase isozymes in chicken-quail hybrid embryos

The ontogeny of lactate dehydrogenase (LDH) isozymes was examined in avian hybrids and compared with the isozyme patterns of the parental species. Hybrids were obtained by crossing female Japanese quali (Coturnix coturnix japonica) with male domestic chickens (Gallus gallus domesticus). By use of starch gel electrophoresis and an enzyme-specific stain, traces of embryonic paternally derived LDH were detected in unincubated hybrid eggs. It was concluded that the embryonic genes coding for the B subunits of LDH are activated during the hours between fertilization and oviposition. In early blastoderms, a great excess of maternally stored LDH is present. In the hybrid, the predominantly maternal pattern of isozymes shifts during embryogenesis to a predominantly paternal pattern. This was considered evidence for differential allelic regulation of LDH inactivation. A progressive trend toward the establishment of the adult distribution of isozymes in various tissues was also observed in the hybrid and quail, and found to be similar to chicken LDH isozyme ontogeny.     

Expression of the paternal genes for lactate dehydrogenase, glutamate dehydrogenase and acetylcholinesterase in the development of hybrid fish between species from the families of Cobitidae and Cyprinidae]

The time of expression of the paternal genes of glutamate dehydrogenase (GDH), lactate dehydrogenase (LDH) and acetylcholine esterase (AChE) was investigated in the development of fish hybrids. The species which differed by the thermostability of homologous enzymes were selected as parental pairs. The appearance of differences in the thermostability of homologous enzymes between the hybrids and the maternal species suggested the beginning of paternal enzyme synthesis in the hybrid embryos. Differences in the AChE thermostability appeared simultaneously with the enzyme activity at the stage of first muscle contractions (35 hrs of development), differences in the mitochondrial GDH thermostability appeared at the stage of hatching (50-60 hrs) and those in the LDH thermostability 12-17 days after hatching. The total activity of AChE and GDH sharply increased during the period of the paternal enzyme appearance whereas the activity of LDH suffered practically no changes. Differences in the AChE thermostability between the hybrids and the maternal species are the same for both the total AChE (in supernatant, 15,000 gX10 min.) and the solubilised AChE (in supernatant, 130,000 gX60 min.). AChE of the parental species and the hybrids have the same electrophoretic mobility. The differences in the thermostability of enzymes are preserved following the electrophoresis in polyacrilamide gel.     

Genetic control of seed proteins in wheat

Summary Electrophoretic profiles of crude protein extracts from seed of F₁ hybrids and reciprocal crosses among diploid, tetraploid and hexaploid wheats were compared with those of their respective parental species. The electrophoretic patterns within each of three pairs of reciprocal crosses, T.boeoticum X T.urartu, T.monococcun X T. urartu and T.dicoccum X T. araraticum, were different from one another but were identical with those of their respective maternal parents. Protein bands characteristic of the paternal parents were missing in F₁ hybrid seed suggesting that the major seed proteins in wheat were presumably regulated by genotype of the maternal parent rather than by the seed genotype. However, in another three pairs of reciprocal crosses, T.boeoticum X T. durum, T.dicoccum X T.aestivum and T. zhukovskyi x T. aestivum, protein bands attributable to the paternal parents were present in the F₁ hybrid seeds indicating that the seed proteins were not always exclusively regulated by the maternal genotype. The expression of paternal genomes is presumably determined by dosage and genetic affinity of the maternal and paternal genomes in the hybrid endosperm. The maternal regulation of seed protein content is probably accomplished through the maternal control over seed size. The seed protein quality may, however, depend upon the extent of expression of the paternal genome.     

Parent‐of‐origin growth effects and the evolution of hybrid inviability in dwarf hamsters

Mammalian hybrids often show abnormal growth, indicating that developmental inviability may play an important role in mammalian speciation. Yet, it is unclear if this recurrent phenotype reflects a common genetic basis. Here, we describe extreme parent‐of‐origin‐dependent growth in hybrids from crosses between two species of dwarf hamsters, Phodopus campbelli and Phodopus sungorus. One cross type resulted in massive placental and embryonic overgrowth, severe developmental defects, and maternal death. Embryos from the reciprocal cross were viable and normal sized, but adult hybrid males were relatively small. These effects are strikingly similar to patterns from several other mammalian hybrids. Using comparative sequence data from dwarf hamsters and several other hybridizing mammals, we argue that extreme hybrid growth can contribute to reproductive isolation during the early stages of species divergence. Next, we tested if abnormal growth in hybrid hamsters was associated with disrupted genomic imprinting. We found no association between imprinting status at several candidate genes and hybrid growth, though two interacting genes involved in embryonic growth did show reduced expression in overgrown hybrids. Collectively, our study indicates that growth‐related hybrid inviability may play an important role in mammalian speciation but that the genetic underpinnings of these phenotypes remain unresolved.     

Lactate dehydrogenase ontogeny,paternal gene activation,and tetramer assembly in embryos of brook trout,lake trout,and their hybrids

Measurement of lactate dehydrogenase in reciprocal hybrids of trout during development revealed that a maternal effect was involved in the regulation of enzyme levels until resorption of the yolk sac was completed. Malate dehydrogenase specific activities were the same in these embryos and larvae. The more negatively charged B subunits of LDH predominated during early stages of embryogenesis in lake trout and brook trout with an increase in synthesis of A subunits evident as development progressed. Activation of the paternal A gene in reciprocal hybrids occurred at a relatively late stage with the LDH subunit specific to the retina appearing after hatching. Analysis of brook trout progeny from a cross of parental types with a variant and wild-type B subunit suggested nonrandom LDH tetramer assembly which may be genetically controlled.This study was supported by National Science Foundation Grant GB-7271.     

Expression of maternal and paternal histone genes during early cleavage stages of the echinoderm hybrid Strongylocentrotus purpuratus × Lytechinus pictus

Interspecific hybrids of the sea urchins Strongylocentrotus purpuratus (♀) and Lytechinus pictus (♂) were used to estimate the contributions of the maternal and paternal genomes to histone mRNA synthesis during early development. Radiolabeled histone mRNAs from the two sea urchin species were identified by hybridization to cloned histone genes from both S. purpuratus and L. pictus and shown to be electrophoretically distinguishable. The synthesis of maternal and paternal histone mRNA in these hybrid embryos is evident as early as the two-cell stage. By at least the 16-cell stage, both maternal and paternal histone mRNAs are associated with polysomes. The relative amounts of the maternal and paternal histone mRNAs synthesized by the zygote appear to be similar.     

Identification of hemiclonal reproduction in three species of Hexagrammos marine reef fishes

Natural hybrids between the boreal species Hexagrammos octogrammus and two temperate species Hexagrammos agrammus and Hexagrammos otakii were observed frequently in southern Hokkaido, Japan. Previous studies revealed that H. octogrammus is a maternal ancestor of both hybrids; the hybrids are all fertile females and they frequently breed with paternal species. Although such rampant hybridization occurs, species boundaries have been maintained in the hybrid zone. Possible explanations for the absence of introgressions, despite the frequent backcrossing, might include clonal reproduction: parthenogenesis, gynogenesis and hybridogenesis. The natural hybrids produced haploid eggs that contained only the H. octogrammus genome (maternal ancestor) with discarded paternal genome and generated F₁‐hybrid type offspring by fertilization with the haploid sperm of H. agrammus or H. otakii (paternal ancestor). This reproductive mode was found in an artificial backcross hybrid between the natural hybrid and a male of the paternal ancestor. These findings indicate that the natural hybrids adopt hybridogenesis with high possibility and produce successive generations through hybridogenesis by backcrossing with the paternal ancestor. These hybrids of Hexagrammos represent the first hybridogenetic system found from marine fishes that widely inhabit the North Pacific Ocean. In contrast with other hybridogenetic systems, these Hexagrammos hybrids coexist with all three ancestral species in the hybrid zone. The coexistence mechanism is also discussed.     

Biochemical genetics of hybrid sunfish: Differential survival of heterozygotes

Allelic segregation in reciprocal backcrosses involving the largemouth bass (Micropterus salmoides) and the F₁ hybrid (largemouth bass × smallmouth bass, M. dolomieui) was investigated to determine the extent of euheterosis and luxuriance. The frequencies of allelic isozymes encoded in the lactate dehydrogenase E, malate dehydrogenase B, and isocitrate dehydrogenase loci were determined for reciprocal backcross progeny subjected to different selection pressures. The progeny of the backcross (male F₁ × female largemouth bass) underwent a rapid loss of heterozygous individuals in a natural pond environment. When the offspring of this same mating were placed in artificial pools, where cannibalism is the main source of mortality, heterozygosity was advantageous. There was a marked correlation of increased heterozygosity at these enzyme loci with an increased growth rate. None of the above responses to selection was observed when the F₁ hybrid served as the maternal parent in the reciprocal backcross. A maternal factor in the egg cytoplasm may influence the expression of heterosis.     

蟾蜍种间核移植胚胎发育早期LDH同工酶的表现

利用聚丙烯酰胺凝胶电泳,对中华大蟾蜍(Bufo bufo比gargarizans)与花背蟾蜍(Bufo raddei)种间核移植胚胎发育早期六个不同阶段中全胚胎的乳酸脱氢酶(LDH)同工酶进行了分析。酶谱比较的结果表明:在正、反种间移核胚胎中,供体核LDH基因的活动开始表现于尾芽胚期;此前,杂种胚胎中LDH同工酶谱类型与受体一致。     

Chloroplast DNA codes for the ribulose diphosphate carboxylase catalytic site on fraction I proteins of Nicotiana species

Summary The specific activity of ribulose diphosphate carboxylase and the K _m for ribulose 1–5 diphosphate of Fraction I protein isolated from N. gossei was significantly higher than enzyme from six other species of Nicotiana which were closely similar to each other. In several interspecific, reciprocal F₁ hybrids, the higher N. gossei activity was present only when N. gossei was the female parent. Consequently, the maternal mode of inheritance indicates chloroplast DNA to contain information which regulates the conformation of the catalytic site on the enzyme. Previous results had shown this DNA to code for the primary structure of the large subunit. In hybrids where N. gossei was the female parent, the specific enzyme activity was intermediate between N. gossei itself and the other parent because of formation of isozymes of Fraction I protein which were demonstrated by tryptic peptide fingerprints of the small subunits of Fraction I protein from N. gossei, N. excelsior, and reciprocal hybrids. Peptides characteristic of the enzyme from each parent were found in both hybrids. Previous results had demonstrated that nuclear genes contain the code for the small subunit. In the hybrid where N. gossei is the female parent, the reduction in specific enzyme activity from that of N. gossei itself is evidently the result of N. excelsior type small subunits modifying the catalytic site on the N. gossei type of large subunits. A hybrid protein of lesser activity is produced which dilutes the higher activity of the N. gossei type protein.