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.

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The proximal third of mouse chromosome 17 is known as the t-complex. The t-haplotype is a variant form of this region containing four tandem inversions compared with the wild-type t-complex, and thus recombination in heterozygotes of the t-haplotype is strongly suppressed along the entire t-complex region. Within this genetically locked t-haplotype, many mutations related to various interesting phenotypes (e.g., taillessness, transmission ratio distortion, recessive lethality) have accumulated, and many mouse geneticists have been attracted to t-haplotype research. Many recessive lethal mutations known as t-complex lethal mutations have been found, and detailed phenotypic analyses have revealed that the functions of t-lethal genes are related to important developmental events. Therefore, identification of the genes responsible for these lethal mutations may contribute to our understanding of the mechanisms of mammalian development. In this review, I introduce the phenotypes of t-lethal mutations and describe recent findings, including our results regarding the molecular identification of a t-lethal gene.

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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.

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We developed polymorphic microsatellite markers in Allium mongolicum Regel, a desert plant widely distributed in western China. To better conserve this species, we need to investigate its genetic diversity and population genetic structure, as well as its evolutionary history. Using the combined biotin capture technique, we isolated 13 novel polymorphic microsatellite loci and evaluated their characteristics using 60 individuals from two populations of A. mongolicum. Two to fourteen alleles per locus were identified for these microsatellites. The observed and expected heterozygosities ranged from 0.132 to 0.875 and 0.447 to 0.986, respectively. These microsatellite markers can be used to assess the genetic variation and genetic structure of A. mongolicum.

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The Indian bullfrog Hoplobatrachus tigerinus and its four congeneric species are common frog species and distributed throughout South Asia. Due to recent human activity, they are facing a changing environment and reduction in natural population size. For effective conservation and molecular ecological studies, we therefore isolated and characterized microsatellite loci for these frogs. We obtained genomic data using an Ion Torrent PGM sequencer and designed 54 primer sets for candidate loci. By screening for polymorphic loci in individuals of H. tigerinus and its congeneric species, we isolated 27 loci as highly polymorphic microsatellite loci. Eight of these loci were commonly applicable for all species except H. chinensis. Within two populations of H. tigerinus, the total number of alleles per locus and expected heterozygosity ranged from 2 to 18 and 0.271 to 0.938, respectively. No significant linkage disequilibrium was observed across all loci, and five showed a significant deviation from Hardy-Weinberg equilibrium in some populations after Bonferroni correction. Consequently, our findings suggest that these novel markers will be applicable for conservation genetic studies across varying scales from inter-population to inter-individual.

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Endogenous bornavirus-like nucleoprotein (EBLN) elements are nucleotide sequences homologous to the bornavirus N gene that have been identified in animal genomes. EBLN elements are considered to have been generated through reverse transcription of bornavirus N mRNA, mainly with the aid of long interspersed element-1 (LINE-1). The genome of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) contains an EBLN element, itEBLN, which is thought to have been integrated less than 8.5 million years ago (MYA). However, it was also reported that the LINE-1 activity on this lineage was lost 4-5 MYA. Here, molecular evolutionary analyses were conducted to gain insights into the integration time of itEBLN. In a phylogenetic analysis of bornavirus N and itEBLN, using an EBLN element from cape golden moles (Chrysochloris asiatica) (caEBLN) as the outgroup, the integration time of itEBLN appeared to be close to the time of the most recent common ancestor (MRCA) for bornavirus N. From an analysis of genomic sequences for bornavirus strains isolated at different time points, the time of the MRCA for bornavirus N was estimated to be < 0.3 MYA. These results suggest that the integration time of itEBLN was much later than the loss of LINE-1 activity, supporting the non-LINE-1-mediated integration of itEBLN.

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This paper was published in error prior to the publication of the gibbon genome sequence assembly and analysis by the Gibbon Genome Analysis Consortium, due to the author’s lack of knowledge of the embargo on the genome. The paper is currently an invalid publication [Corrigendum; Genes Genet. Syst. 87: 277 (2012)].
The gibbon genome paper was published on Sept. 11, 2014 (1). The authors asked the Consortium for amending the status of the publication and make it valid again. This request was approved by the Consortium on October 10, 2014. The paper can be regarded as a valid publication effective of October 10, 2014.

October 10, 2014
Akihiko Koga, Corresponding author

Citation:
(1) Carbone et al. (2014) Gibbon genome and the fast karyotype evolution of small apes. Nature 513, 195–201. doi: 10.1038/nature13679

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