Postzygotic reproductive isolation is the reduction of fertility or viability in hybrids between genetically diverged populations. One example of reproductive isolation, hybrid male sterility, may be caused by genetic incompatibility between diverged genetic factors in two distinct populations. Genetic factors involved in hybrid male sterility are disproportionately located on the X chromosome. Recent studies showing the evolutionary divergence in gene regulatory networks or epigenetic effects suggest that the genetic incompatibilities occur at much broader levels than had previously been thought (e.g., incompatibility of protein-protein interactions). The latest studies suggest that evolutionary divergence of transcriptional regulation causes genetic incompatibilities in hybrid animals, and that such incompatibilities preferentially involve X-linked genes. In this review, we focus on recent progress in understanding hybrid sterility in mice, including our studies, and we discuss the evolutionary significance of regulatory divergence for speciation.
The insect steroid hormone ecdysteroid plays pivotal, roles in the temporal coordination of development, represented by molting and metamorphosis. During the larval stages, ecdysteroid is biosynthesized from dietary cholesterol by several ecdysteroidogenic enzymes in the specialized endocrine organ called the prothoracic gland (PG). As ecdysteroid biosynthesis in the PG is affected by several environmental cues, such as photoperiod and nutrition, a fundamental question is how the ecdysteroid biosynthesis pathway is controlled in response to environmental cues. In this review, we briefly summarize recent topics on the regulatory mechanisms of ecdysteroid biosynthesis, especially the neuronal regulatory mechanism, in the fruit fly Drosophila melanogaster. The implications from studies with other insects are also discussed.
Domestication-related changes that govern a spike morphology suitable for seed harvesting in cereals have resulted from mutation and selection of the genes. A synthetic hexaploid wheat (S-6214, genome AABBDD) produced by a cross between durum wheat (AABB) and wild goat grass (DD) showed partial non-domestication-related phenotypes due to genetic effects of the wild goat grass genome. Quantitative trait loci (QTLs) affecting wheat domestication-related spike characters including spike threshability, rachis fragility and spike compactness were investigated in F2 progeny of a cross between Chinese Spring (CS) wheat (AABBDD) and S-6214. Of 15 relevant QTLs identified, eight seemed to be consistent with peaks previously reported in wheat, while four QTL regions were novel. Four QTLs that affected spike threshability were localized to chromosomes 2BS, 2DS, 4D and 5DS. The QTL on 2DS probably represents the tenacious glume gene, Tg-D1. Based on its map position, the QTL located on 2BS coincides with Ppd-B1 and seems to be a homoeolocus of the soft glume gene. Two novel QTLs were detected on 4D and 5DS, and their goat grass alleles increased glume tenacity. Three novel QTLs located on 2DL, 3DL and 4D for rachis fragility were found. Based on the map position, the QTL on 3DL seems different from Br1 and Br2 loci and its CS allele appears to promote the generation of barrel-type diaspores. Three disarticulation types of spikelets were found in F2 individuals: wedge-type, barrel-type and both types. Among eight QTL peaks that governed spike morphology, six, located on 2AS, 2BS, 2DS, 4AL and 5AL, coincided with ones previously reported. A QTL for spike compactness on 5AL was distinct from the Q gene. A novel QTL that controls spike length was detected on 5DL. Complex genetic interactions between genetic background and the action of each gene were suggested.
To achieve the precise wiring of axons in the brain required to form a fine architecture, a molecular level interaction between axons and their targets is necessary. The Drosophila visual system has a layered and columnar structure which is often found in the brain of vertebrates. With powerful genetic tools for its analysis, the Drosophila visual system provides a useful framework to examine the molecular mechanisms of axon targeting specificity. The medulla is the second optic ganglion in the Drosophila optic lobe, and is subdivided into ten layers. Among the eight photoreceptor types, R7 and R8 pass through the first optic ganglion lamina and innervate the medulla. In the medulla, R7 and R8 axons grow in a distinct manner to reach their final target layers: M6 and M3, respectively. The axons from R7 and R8 take characteristic steps to extend toward their target layer. In this review, we discuss the formation of the Drosophila optic lobe and the molecular mechanisms of layer specific targeting of R8 axons in the medulla. Fundamental and comprehensive understanding of the crosstalk of growing axons and target regions in the Drosophila optic lobe will elucidate the general principles applicable to more complex nervous systems.
Genetic variations of functional genes in various animal species have been previously examined to understand the relationship between genotypes and phenotypes. Repeat polymorphism can be found in not only coding sequences but also untranslated regions of the protein-coding genes, affecting gene regulation via a variety of mechanisms. In this respect, repeat polymorphisms including microsatellites, variable number of tandem repeats (VNTRs), and short interspersed nuclear elements (SINEs) in relation to functional genes contribute to recognition of genetic or phenotypic variation and individual identification. These elements are biologically valuable markers for studies on association between genetic make-up and behavioral patterns in dog breeds. Hence, to examine the effect of genotype on behavior, studies on this combination are certainly important. Hence, this review could cast light on the functional roles of repeat polymorphisms in dog behavior and breed variation.
In order to elucidate the nucleotide sequence divergence of mitochondrial DNAs between two Japanese pond frog species Rana nigromaculata and R. brevipoda, and the mode of inheritance of cytoplasmic genomes in the female hybrid lineages of the two species, the cleavage patterns of mtDNAs digested with 10 restriction endonucleases were examined by agarose gel electrophoresis using a total of 52 frogs including Rana nigromaculata and R. brevipoda, their reciprocal hybrids and the backcross offspring (B-1 and B-2) derived from female hybrids by crossing with the paternal species. The cleavage patterns for mtDNA of Rana nigromaculata were different from those of R. brevipoda digested with ail the restriction endonucleases used except EcoRV. The nucleotide sequence divergence of mtDNAs between these two species was roughly estimated to be 8.5%. The cleavage patterns for mtDNAs of the reciprocal hybrids and the B-1 and B-2 offspring were clearly similar to those of the maternal species, and paternal mtDNAs could not be detected. On the other hand, the proportions of original maternal nuclear genes at the 22 allozyme loci were 50% in the reciprocal hybrids, 21.9% or 25.6% in the B-1 offspring, and 7.5% in the B-2 offspring. These results demonstrate that nuclear genomes decrease the original maternal constitution in the female hybrid lineages generationally, whereas the mtDNAs are inherited maternally during repeated backcrossing.
The chromosomal locations of isozyme gene loci encoding isocitrate dehydrogenase (IDH) and phosphoglucoisomerase (PGI) were investigated in shallot (Allium cepa L. Aggregatum group). The alien monosomic addition lines of A. fistulosum L. with the extra chromosomes (1A, 2A, 5A, 6A, 8A and other unidentified chromosomes) from A, cepa Aggregatum group were used as plant materials. Both gene loci, Idh-1 and Pgi-1, were located on the chromosome 5A. The present study established two new isozyme markers for the chromosome 5A in addition to the six previously reported isozyme markers for the chromosomes 1A, 2A, 6A and 8A.
Erythropoietin stimulates proliferation and differentiation of erythroid progenitor cells by binding to a specific membrane receptor, erythropoietin receptor. By using the genomic clone derived from a rat cosmid library, the rat erythropoietin receptor gene was assigned to chromosome 8q24 by fluorescence in situ hybridization.
In order to investigate the intraspecific variation of Prunus yedoensis (Someiyoshino) and interspecific relationship among P. yedoensis, P. lannesiana (Oshimazakura) and P. pendula (Edohigan), DNA fingerprinting study was conducted by using two different kinds of probes, M13 repeat sequence and (GACA)(4) synthetic oligonucleotide. In this study, 68 plants of P. yedoensis grown in 46 prefectures in Japan were investigated. All the P. yedoensis individuals investigated showed the completely same banding pattern, indicating their clonal origin from a single plant. On the other hand, each of P. lannesiana and P. pendula individuals investigated showed a unique banding pattern, suggesting a considerable amount of genetic variation in these two species. About 90% of bands in DNA fingerprints of P. yedoensis were detected in either P. lannesiana or P. pendula. This result supports the hypothesis that P. yedoensis is an interspecific hybrid between P. lannesiana and P. pendula. From those results, it is concluded that P. yedoensis was produced only once through hybridization between P. lannesiana and P. pendula, and that this particular hybrid plant has been spread vegetatively all over Japan.
To estimate approximate times of divergence of animal phyla lacking fossil data, it is important to find a molecule that evolves with an approximately constant rate over a wide evolutionary distance covering the whole animal phyla. For this purpose, the evolutionary rate constancy has been examined for 20 proteins. It was found that four proteins, particularly the aldolase C, involved in the glycolitic pathway, had evolved with rates that are approximately constant not only among different classes of vertebrates, but also between vertebrates and arthropods. The evolutionary rate (= 0.26 x 10(-9)/site/year) of the aldolase C is likely to have remained essentially unchanged even between animals and plants.
The chromosomal locations of isozyme gene loci encoding leucine aminopeptidase (LAP), glutamate-oxaloacetate transaminase (GOT), 6-phosphogluconate dehydrogenase (6-PGDH), alcohol dehydrogenase (ADH) and glutamate dehydrogenase (GDH) were investigated in shallot (Allium cepa L. Aggregatum group). The alien monosomic addition lines of A. fistulosum L. with the extra chromosomes (1A, 2A, 6A, 8A and other unidentified chromosomes) from A. cepa Aggregatum group were used for plant materials. The results revealed that the five gene loci, Lap-1, Got-1, 6-Pgdh-2, Adh-1 and Gdh-1, were located on the chromosomes 1A, 2A, 2A, 6A and 8A, respectively.
cDNA corresponding to two hsp70-related genes (OLHSC70 and CEHSC70) were isolated from two lines of cultured fish cells derived from the genus Oryzias. OLHSC70 was 2,261 bp in length and encoded a protein of 686 amino acids with a predicted molecular mass of 76,120 daltons. CEHSC70 was 2,114 bp in length and it lacked the 5' region found in OLHSC70. Two-dimensional electrophoresis revealed that Oryzias latipes has at least three heat-inducible proteins with molecular masses of about 70,000 daltons. One of these proteins (Hsp70.1) was barely expressed under normal conditions but its high-level expression was induced by hyperthermia. The other two proteins (Hsc70.1, and Hsc70.2) were constitutively expressed under normal conditions and only slightly enhanced levels were induced by hyperthermia. Transfection with the cloned sequence, RNA dot-blot analysis and the two-dimensional electrophoresis of proteins showed that OLHSC70 encoded Hsc70.1.
CA-repeat primed polymerase chain reaction (CAP-PCR), using degenerate primers which anneal at the ends of (CA)n sequences in eukaryotic genomes, was attempted to assess its potential to monitor the genomic polymorphisms in various animals. Three mammalian, three avian, one fish and one insect species were examined and all showed primer-specific DNA fingerprints by CAP-PCR. Polymorphic bands observed in a laboratory-bred vole family were segregated in Mendelian manner. The present CAP-PCR DNA fingerprinting therefore is a simple and useful method for examining genomic variations in most animals without prerequisite knowledge of DNA sequences.
Mitochondrial DNAs (mtDNAs) were purified by CsCl/bisbenzimide density-gradient ultracentrifugation from 21 strains of seven Fusarium species that cause fusarium head blight and mycotoxin contamination in wheat and other cereals. A partial PstI clone bank, from which one of twelve PstI fragments (14.7 kb) is missing, was constructed using mtDNA from strain KU-1615 of F. graminearum. Molecular sizes of mtDNAs of single representative strains from the seven species were determined after single-, double- and triple-digestion by four or five restriction enzymes (BamHI, MluI, PstI, PvuII and XhoI), while those of others were after single-digestion by BamHI and/or PstI. MtDNA size varied from the smallest 49 kb in one strain of F. avenaceum to the largest 116 kb in one strain of F, culmorum. Restriction fragment length polymorphism (RFLP) analysis revealed a large interspecific variation, thus all the species were identified by their restriction fragment patterns and assigned to individual clusters except for F. tricinctum in that a strain studied showed identical patterns to one of two strains of F. sporotrichioides. Considerable intraspecific variation including size variation was also detected. These results indicated a high incidence of insertions/deletions both between and within species. On the basis of results obtained by the cluster analysis, some aspects of taxonomy in these Fusarium species were discussed.
Using DNA sequence data of 18 genes from 14 mammals, we analyzed how the average molecular evolution rate per year per site (Vy) depends on the generation time (g). (I) Assuming the relation Vy varies; is directly proportional to g(-alpha), the index of generation time effect, (alpha) was estimated to be about 0.14 for amino acid replacement substitutions (A), and about 0.32 for synonymous substitutions (S). (II) Assuming the relation Vy = V(m)g g-1 + V(e)y, where V(m)g and V(e)y are constant independent of g, the fraction, r(e) = V(e)y/Vy, of the mutation rate independent part (V(e)y) in the total evolution rate (Vy) was estimated under the assumptions of the star phylogeny and the constancy of the mutation rate per generation. r(e) was smallest for mouse with the shortest generation time among our analyzed species, and it was estimated to be about 0.57 for A and 0.31 for S. Both results do not support the view that Vy is equal to the neutral mutation rate per site both for A and for S. They are in line with the thesis that, at least for A and probably even for S, the molecular evolution rate is influenced by some causes other than the mutation rate, such as changing environment.
Four kinds of circular plasmid-like DNA, designated B1, B2, B3 and B4, have been found in the mitochondria of Oryza sativa L. with an AA genome. Three novel B1-homologous mitochondrial plasmid-like DNAs, designated, M1, M2 and M3, were isolated in the present study from strains with CC and CCDD genomes in the genus Oryza. We cloned and sequenced these DNAs and found that the sequences of these molecules have wide regions of homology. B1, M2 and M3 each lack about 300 bp of a region that is present in M1 and small repeats were found at the sites of deleted sequences. Therefore, we propose the hypothesis that the B1 family differentiated from a common ancient molecule that was similar to M1 via, probably, slipped mispairing during DNA replication at several stages in the evolution in the genus Oryza.
The tef-1 gene encoding translation elongation factor 1 alpha was cloned from the ascomycete fungus Neurospora crassa. The sequences of genomic DNA and cDNA clones showed that the tef-1 gene contained one ORF of 1380 bp length that is interrupted by three short introns. The deduced polypeptide contained 460 amino acid residues, and the sequence had a high similarity with those of EF-1 alpha polypeptides from other species. The level of tef-1 mRNA was low in conidia but high in growing cells. When mycelia were transferred to poor nutrient media, the level of tef-1 gene mRNA decreased remarkably. The pattern of tef-1 expression was similar to the expression of genes for ribosomal proteins. The tef-1 gene was mapped between arg-3 and leu-4 loci on linkage group I by restriction fragment length polymorphism mapping. Southern blot analysis showed that Neurospora genomic DNA contained only one copy of the tef-1 gene in a genome.
The musk shrew (Suncus murinus) is widely distributed throughout Asia and East Africa. The mitochondrial DNA (mtDNA) of this species was analyzed in individuals from 31 local populations in nine Asian countries and Mauritius, using 17 restriction endonucleases. Although fourteen and nine mtDNA haplotypes were detected from Bangladesh and Nepal, respectively, one to four haplotypes were found in each Southeast Asian country, and one common haplotype existed in Japan, Philippines, Vietnam, Thailand and Indonesia. Clustering analysis of mtDNA haplotypes classified shrew populations to three groups-continental group (Bangladesh and Nepal), islands' group (insular countries and Vietnam) and Malay group. The average nucleotide diversity among these three groups was calculated to be about 3.5%. These results indicate that the origin of feral populations in this species might be old and their population sizes are extremely large in the continent, and suggest a rapid spread of this animal throughout the islands. Although we have not shown yet an evidence of close relationships between islands' and continental mtDNA haplotypes, it is likely that the musk shrew had migrated from the continent in South Asia to the islands in Southeast Asia recently.
