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.     

Growth and reproduction of two species of Akodon and their hybrids

Growth and reproduction of Akodon molinae (A.m.), A. dolores (A.d.) (Rodentia: Cricetidae), and their hybrids are described. A.d. showed less fertility under animal room conditions, fewer "successful" matings, similar litter sizes, and an altered sex proportion as compared to A.m.; A.d. ovulatory index is supposed to be higher than mean litter size. X-linked lethal gene(s) and chromosome polymorphisms are possible causes of these variations. Interspecific crosses showed a marked seasonal (summer) tendency in parturitions. Intraspecific matings also produced young during winter. F1 hybrids showed a reversed tendency, while F2 matings returned to the parental pattern. A.d. male X A.m. female and F1 crosses presented larger litter sizes than the other pairings. A.m. exhibited sexual differences in weight after 60 days of age. A.d. and F1 hybrids did not show significant differences in weight of both sexes at any age. A.m. males and females were heavier than A.d. individuals except 12 months after birth. F1 hybrids born to A.m. male were heavier at birth than the A.d. female s interspecific offspring, and may be interpreted as a maternal influence of the A.m. females. Hybrids were heavier than their parents at birth and some of them were also heavier at 21 days, and probably is due to pseudoheterosis. The comparison of body measurements between sexes of both species did not give conclusive results. A.m. females were larger than females of A.d. Males of both species showed similar total lengths; A.m. males had larger body lengths and shorter tails. Body measurements were not related to weight variations. However, differences in weight and measurements between sexes appeared simultaneously at 2-6 months. This probably reflected the attainment of maturity.     

Gene regulatory divergence among species estimated by altered developmental patterns in interspecific hybrids

Disturbances in the schedules of gene expression in developing interspecific fish hybrids have been used to draw inferences about the extent of gene regulatory divergence between species and about the degree to which this gene regulatory divergence is correlated with structural gene divergence, as estimated by genetic distance. Sperm from each of 10 different species representing six genera within the family Centrarchidae was used to fertilize eggs of the Florida largemouth bass (Micropterus salmoides floridanus). The genetic distances (D; Nei 1978) between the parental species used to form the hybrids ranged from 0.133 to 0.974. The developmental success and temporal patterns of gene expression of each of the hybrids were compared with those of the Florida largemouth bass. As the genetic distance between the paternal species and the Florida largemouth bass increased, there was a general decline in developmental success in the hybrid embryos as demonstrated by the observed reductions in the percentage of hatching and by progressively earlier and more extensive morphological abnormalities. Concomitantly, progressively more marked alterations in developmental schedules of expression of 15 enzyme loci occurred in the hybrids as the genetic distance between parental species increased. However, observed deviations from this trend for a few species may represent an uncoupling of the rates and modes of evolution of structural genes from those for genes regulating developmental processes.      

Developmental capacity of aneuploid Xenopus species hybrids.

Diploid as well as triploid Xenopus interspecific hybrids generate aneuploid eggs because of the presence, at meiosis, of univalent chromosomes which are presumably distributed at random. Zygotes obtained from such eggs, fertilized by either normal or UV-irradiated sperm, were analysed for their developmental capacities. All monosomics die in the course of embryogenesis, whereby optimum capacities correspond closely with those observed in monosomic mammalian embryos, especially in mice. In contrast, hyperdiploid Xenopus are relatively viable: although many die exhibiting the 'haploid syndrome' or various other abnormalities, 8% of them reach metamorphosis, and 1-2% become adults. Of the latter, the karyotype was established in 13 individuals. Among them, 8-16 supernumerary chromosomes were found to be present.     

Developmental characteristics of two closely related species of Allonemobius and their hybrids

Summary Two species of ground crickets, Allonemobius fasciatus (from Corvallis, 44.3 N) and A. socius (from Gainesville, 30.3 N), and their hybrids were compared for nymphal development, wing form, and embryonic diapause. A. fasciatus matured more rapidly at a short photoperiod than at a long photoperiod and also than A. socius which showed no developmental response to photoperiod. The proportion of long-winged forms varied with nymphal photoperiod greatly in A. fasciatus but slightly in A. socius. A. fasciatus laid only diapause eggs, while A. socius responded to a long parental photoperiod and a high incubation temperature by averting diapause. A. fasciatus showed temperature-dependent shift of the diapause stage butA. socius entered diapause only at the early stage. These differences are related to their voltinism and distribution.Reciprocal crosses between the two species produced viable hybrids. In development rate, female hybrids were intermediate between their parents while male hybrids were closer to their mothers, suggesting that the development rate is controlled primarily by the X chromosome. Overdominance occurred in wing form; reciprocal crosses produced either higher or lower proportions of long-winged forms than their parents. Diapause of F1 hybrid embryos was determined only by the female parent and the paternal effect appeared in F2 embryos.     

Expression of the lactate dehydrogenase genes in Xenopus species hybrids

The lactate dehydrogenase isozyme patterns of six Xenopus species-hybrid combinations have been investigated. The genes from both parental species were equally expressed in all of them. “Hybrid” isozymes originating from aggregation of subunits from both parents could be visualized and demonstrated random aggregation of enzyme subunits. Reciprocal crosses displayed the same pattern demonstrating also random aggregation of subunits.     

Levels of mate recognition within and between two Drosophila species and their hybrids

If sexual selection is to result in speciation, traits involved in mate choice within species need to be capable of producing sexual isolation between species. We investigated the association between mate choice and sexual isolation using interspecific hybrids between two sibling species, Drosophila serrata and Drosophila birchii. A perfuming experiment demonstrated that olfaction was involved in the sexual isolation between the two species. A quantitative genetic analysis using 30 populations of hybrids between the two species indicated that mating success in hybrid individuals was predominately determined by cuticular hydrocarbons; the average genetic correlation between mating success and cuticular hydrocarbon profile was 0.84, and in some instances exceeded 0.95. Multivariate analysis of the cuticular hydrocarbons of the two species revealed that there were three independent blends of cuticular hydrocarbons that separated three levels of organization: species, sex, and sex within species. The hydrocarbons used by hybrids in mate choice included those that separated the two species, demonstrating that species-specific characters may be used in mate choice within populations. The interspecific reciprocal cross had major effect on which cuticular hydrocarbons were associated with mating success, indicating that the expression of the cuticular hydrocarbons was strongly sex linked.     

Genomic compatibility between two phyllotine rodent species evaluated through their hybrids

In order to investigate the genomic compatibility between allopatric rodent species, Phyllotis darwini and Phyllotis magister, we have studied several cytogenetic and reproductive features of their laboratory hybrids. Of thirty-one pairings between species, only five were successful, producing eleven newborns. Like parents, hybrids had 38 metacentric chromosomes, except for the subtelocentric Y chromosome inherited from P. magister. There was almost total C and G band correspondence between homeologous autosomes. However, parental sex chromosomes had different morphology, C and G bands. Ag-NOR bands appeared as small telomeric Ag+ regions, distributed in four chromosomal pairs of darwini, three of magister and four homeologous chromosomes of the hybrids. The three forms had similar indexes of NOR activity per cell, in spite of the variability in NOR expression which was always detected. Usually, only one member of parental homologous chromosomes showed AgNOR+; nevertheless, both homeologous chromosomes were active in many hybrid cells. The frequencies of cells that expressed their ribosomal genes in the two homologous or homeologous NOR chromosomes were similar in parental and hybrid cells. These results strongly suggest that ribosomal genes of both parental genomes would function codominantly in the hybrids. The gonad histological and morphometric analyses showed that hybrids conformed to Haldane's rule, since females were fertile and males were infertile. Our results indicate that P. darwini and P. magister genomes can function in relative harmony and compatibility when they are placed together in their laboratory generated hybrids, suggesting that these species have few genetic differences, probably because they have recently diverged.     

Mitochondrial DNA patterns are similar in gametosomatic and somatic hybrids of two Nicotiana species

Summary The segregation and recombination patterns of mitochondrial genome in the somatic hybrids of Nicotiana tabacum and N. rustica were studied by RFLP analysis using four heterologous mitochondrial DNA probes, namely cytochrome oxidase subunit I (COI), cytochrome oxidase subunit II (COII), 26s rDNA and 5s-18s rDNA. These RFLP patterns were compared with those of the gametosomatic hybrids of these two species. A preponderance of N. rustica type patterns was observed in the somatic hybrids. One of the somatic hybrids had N. rustica type pattern with COI probe, novel pattern with COII, and 26s rDNA probe and N. tabacum type pattern with 5s-18s rDNA probe. These patterns are identical to those of some of the gametosomatic hybrids and could only be due to the recombination of mitochondrial genomes of the two parents. The extent and the nature of recombination of mitochondrial genomes is similar in gametosomatic and somatic hybrids.     

Chromosomal differentiation in two species of Aedes and their hybrids revealed by giemsa C-banding

Giemsa C-banding patterns in two species of mosquitoes, Aedes aegypti and Aedes mascarensis, their hybrids and backcross progeny revealed differences in the sex chromosome pair. In A. aegypti, the female determining or the m chromosome in both males and females shows a conspicuous band in the centromere region and another band in one arm. The male determining or the M chromosome is devoid of any bands. Progeny of crosses involving A. aegypti females and A. mascarensis males showed interesting albeit unexpected results. The intercalary band was suppressed in both sons and daughters. When such F₁ sons were backcrossed to A. aegypti females, a proportion of males developed into intersexes. These intersex progeny differed from the normal males in terms of their banding pattern. In the reciprocal cross (A. mascarensis female × A. aegypti male), the F₁ and the backcross progeny yielded the expected C-banding patterns. The implications of the reversible expression of the intercalary band on the A. aegypti m chromosome and its relevance to genetic regulation are discussed.     

Genetic variation and phylogeographic analyses of two species of Carpobrotus and their hybrids in California

Despite the commonality and study of hybridization in plants, there are few studies between invasive and noninvasive species that examine the genetic variability and gene flow of cytoplasmic DNA. We describe the phylogeographical structure of chloroplast DNA (cpDNA) variation within and among several interspecific populations of the putative native, Carpobrotus chilensis and the introduced, Carpobrotus edulis (Aizoaceae). These species co-occur throughout much of coastal California and form several 'geographical hybrid populations'. Two hundred and thirty-seven individuals were analysed for variation in an approximate 7.0 kb region of the chloroplast genome using PCR-RFLP (polymerase chain reaction - restriction fragment length polymorphism) data. Phylogenetic analyses and cpDNA population differentiation were conducted for all morphotypes. Historic geographical dispersion and the coefficient of ancestry of the haplotypes were determined using nested clade analyses. Two haplotypic groupings (I and II) were represented in C. chilensis and C. edulis, respectively. The variation in cpDNA data is in agreement with the previously reported allozyme and morphological data; this supports relatively limited variation and high population differentiation among C. chilensis and hybrids and more wide-ranging variation in C. edulis and C. edulis populations backcrossed with C. chilensis. C. chilensis disproportionately contributes to the creation of hybrids with the direction of gene flow from C. chilensis into C. edulis. The cpDNA data support C. chilensis as the maternal contributor to the hybrid populations.     

Rice histone deacetylase genes display specific expression patterns and developmental functions

Histone deacetylases (HDAC) are important in plant gene expression. Here we show that the expression of rice HDAC genes is both tissue/organ-specific, and most of them are responsive to drought or salt stresses. Over-expression of several rice HDACs did not produce any visible phenotype, whereas down-regulation of a few HDAC genes affected different developmental aspects. Specifically, down-regulation of HDA703 by amiRNA reduced rice peduncle elongation and fertility, while inactivation of a closely related homolog HDA710 by RNAi affected vegetative growth. HDA704 RNAi altered plant height and flag leaf morphology. Down-regulation of HDT702 led to the production of narrowed leaves and stems. These data suggest that rice HDAC genes may have divergent developmental functions compared with closely related homologs in Arabidopsis.     

Organization of Xenopus histone gene variants within clusters and their transcriptional expression

Using previously cloned Xenopus nucleosomal core histone genes as hybridization probes, a genomic DNA library of Xenopus laevis was screened for histone gene clusters. From over 200 histone-gene containing clones identified, 36 were selected as possibly containing H1 histone genes by hybridization to a probe derived from a sea urchin H1 histone gene. These 36 clones were further analyzed by hybrid-selected translation for the definitive presence of H1 histone genes. The genes for three different H1 histone variants were found: H1A , H1B and H1C . Mapping of the histone genes within each clone showed that at least three different gene arrangements can occur within a cluster and that the type of H1 histone variant present in a cluster may be related to the cluster type. S1-mapping experiments indicated that histone genes found in different cluster-types can be expressed in oocytes. Also, the H1 gene found in one cluster-type was expressed in at least three different cell-types: oocytes, gastrula-stage embryos, and erythroblasts.     

A study of the properties of hybrids of oxyhaemoglobin and deoxyhaemoglobin with two porphyringlobin species

The fluorescence of porphyringlobin is quenched on adding haemoglobin to its solutions. It is suggested that this result indicates the formation of hybrids (comprising a dimer of porphyringlobin and a dimer of haemoglobin) in which quenching occurs by energy transfer from the porphyrin to the haem groups of the protein. From an analysis of fluorescence quenching, dissociation constants were calculated for the hybrids of oxy- and deoxyhaemoglobin with the fast- and slow-moving porphyringlobin species isolated by chromatography on CM-Sephadex (Treffry & Ainsworth, 1974). The values obtained are: deoxyhaemoglobin-fast-moving porphyringlobin, 0.8x10(-9)m; deoxyhaemoglobin-slow-moving porphyringlobin, 5x10(-10)m; oxyhaemoglobin-fast-moving porphyringlobin, 0.8x10(-6)m; oxyhaemoglobin-slow-moving porphyringlobin, 1.2x10(-7)m. The rates of reactions of solutions of haemoglobin and porphyringlobin, containing hybrids, with the thiol reagent 4,4'-dithiodipyridine showed that the thiol groups of the hybrids deoxyhaemoglobin-fast-moving porphyringlobin and oxyhaemoglobin-slow-moving porphyringlobin react more slowly than expected on the basis of composition alone: this result indicates that the deoxy and slow-moving conformations are the more stable, imposing themselves partially on to the fast-moving or oxy dimer of the hybrid. Also the rate of the reaction of CO with deoxyhaemoglobin is decreased when slow-moving porphyringlobin is added to its solutions: this is reflected in a movement of the oxygen equilibrium curve of such a mixture to higher oxygen partial pressures. Similar experiments with deoxyhaemoglobin solutions containing fast-moving porphyringlobin, showed an initial increase in the rate of CO uptake. Correspondingly, the oxygen equilibrium curve of the mixture showed an increased affinity for oxygen. Approximate calculations to determine the oxygen equilibria of the hybrids indicate that a functional dimer retains co-operative characteristics even when the dimer accompanying it within the tetramer has the reacted conformation.