Research into the Strength of an Open Wagon with Double Sidewalls Filled with Aluminium Foam

The research is concerned with the use of double walls filled with aluminium foam for an open wagon in order to decrease the dynamic stresses during the operational modes. The research presents the strength calculation for the bearing structure of an open wagon with consideration of the engineering solutions proposed. It was found that the maximum equivalent stresses appeared in the bottom section of the centre sill behind the back support; they amounted to about 315 MPa and did not exceed the allowable values. The maximum displacements were detected in the middle section of the centre sill and amounted to 9.6 mm. The maximum deformations were 1.17 × 10⁻². The research also presents the strength calculation for a weld joint in the maximum loaded zones of the bearing structure of an open wagon and gives the results of a modal analysis of the bearing structure of the improved open wagon. It was found that the critical oscillation frequencies did not exceed the allowable values. The results of the research may be useful for those who are concerned about designing innovative rolling stock units and improving the operational efficiency of railway transport.     

From the Cover: Mass of genes rather than master genes underlie the genomic architecture of amphibian speciation

The genetic architecture of speciation, i.e., how intrinsic genomic incompatibilities promote reproductive isolation (RI) between diverging lineages, is one of the best-kept secrets of evolution. To directly assess whether incompatibilities arise in a limited set of large-effect speciation genes, or in a multitude of loci, we examined the geographic and genomic landscapes of introgression across the hybrid zones of 41 pairs of frog and toad lineages in the Western Palearctic region. As the divergence between lineages increases, phylogeographic transitions progressively become narrower, and larger parts of the genome resist introgression. This suggests that anuran speciation proceeds through a gradual accumulation of multiple barrier loci scattered across the genome, which ultimately deplete hybrid fitness by intrinsic postzygotic isolation, with behavioral isolation being achieved only at later stages. Moreover, these loci were disproportionately sex linked in one group (Hyla) but not in others (Rana and Bufotes), implying that large X-effects are not necessarily a rule of speciation with undifferentiated sex chromosomes. The highly polygenic nature of RI and the lack of hemizygous X/Z chromosomes could explain why the speciation clock ticks slower in amphibians compared to other vertebrates. The clock-like dynamics of speciation combined with the analytical focus on hybrid zones offer perspectives for more standardized practices of species delimitation.

Reproductive isolation (RI) is the cornerstone of speciation—the evolution of diverging populations into separate species. RI is a multidimensional process, influenced by a complex combination of genetic, behavioral, and ecological factors (1), in which postzygotic barriers—the reduction of hybrid fitness by lower fertility or survival—play a decisive part (2, 3). Understanding how and when these barriers prevent hybridization, lineage merging, and ultimately determine speciation versus despeciation is a fundamental topic in evolutionary biology, with important consequences for the cataloging of biodiversity. Although the genetic bases of RI have been under intensive focus, many fundamental questions remain unanswered, including how many loci are needed to generate new species (4–6).Two competing answers have been suggested in the speciation genetics literature. On the one hand, speciation may start when hybridization is strongly reduced by just a few genes with large effects on hybrid fitness (i.e., “master genes” of speciation), affecting key reproductive, behavioral, and ecological traits (7, 8). RI builds up faster when natural selection acts on a handful of speciation genes concentrated in few genomic regions (8–12), even more so if these are linked by reduced recombination as in inversions (13, 14). On the other hand, postzygotic isolation may be initiated gradually by multiple minor incompatibilities, such as Bateson–Dobzhansky–Muller interlocus epistatic incompatibilities that randomly accumulate across the entire genome as lineages diverge (15–17). After a certain point, the multiplying effects of this growing “mass of genes” erodes hybrid fitness (17–22). These two views imply different expectations regarding the relationship between RI and genetic divergence. Small-effect incompatibilities should gradually increase with divergence time (at least initially), while a few major-effect incompatibilities can be set off any time after divergence is initiated.Another burning question in speciation genetics is whether sex chromosomes are hotspots for barrier loci, which in turn can illuminate on the evolutionary forces underlying hybrid incompatibilities (23). The role of sex chromosomes has been popularized by the two empirical “rules of speciation” (i.e., Haldane’s rule and the large X-effect). Haldane’s rule denotes that when a sex is absent, rare, or sterile in an interspecific cross, it is usually the heterogametic sex (24), which has been verified in many animals (25). The large X-effect refers to the disproportionally high impact of X or Z chromosomes in driving hybrid dysfunctions compared to autosomes (26). Initially identified in Drosophila (4, 27), this rule has been verified by restricted introgression at sex-linked loci across many hybrid zones of mammals (28), birds (29), and insects (30).It has become widely accepted that Haldane’s rule and the large X-effect are primarily caused by hemizygosity on the X/Z chromosomes (due to the decayed nature of the Y/W), as they express recessive incompatibilities in the heterogametic sex (25, 31). Hence, one can predict that organisms whose gametologs have not degenerated, like most amphibians and fishes, should not follow these rules of speciation. Yet this prediction has rarely been explored empirically because of the difficulty of identifying morphologically similar (homomorphic) sex chromosomes (32, 33). Without the confounding effects of hemizygosity, the large X-effect then becomes an ad hoc test to assess the relative contribution of sex-linked genes in driving incompatibilities with alternative mechanisms (34), namely the faster evolution of male-expressed genes (“faster male” hypothesis, refs. 35–38) and of interacting X–Y genes necessary for the development of the heterogametic sex (“faster heterogametic sex” hypothesis, refs. 39, 40).The genetic architecture of RI has been traditionally inferred from quantitative trait loci (QTL) mapping of incompatibilities segregating among hybrid progenies, as obtained from interspecies experimental crosses (4). This is only feasible in captive-bred organisms with a short generation time and, in practice, remains limited to a few laboratory model organisms (notably Drosophila). Furthermore, this approach lacks informativeness when RI is strongly polygenic, because genes of small individual effects are difficult to pinpoint. For less-accessible species, speciation researchers have screened for peaks of genome divergence between species pairs (41), which may be associated with RI (e.g., genes resisting introgression) and interspecies differentiation (e.g., genes evolving faster than the genome average). However, the landscape of divergence between closely related genomes does not depend only on their permeability to interspecific gene flow but also on intragenomic variation in recombination and mutation rates, which affects the level of background selection and thus diversity (42, 43). Direct approaches are therefore required to identify genomic regions that resist admixture because they harbor the genes involved in RI (6).As an alternative to traditional speciation genetic studies, we addressed these fundamental questions by directly measuring the permeability of genetic barriers at various stages of divergence across natural species boundaries. We set up a multilevel comparative framework of hybrid zones between anuran amphibians from the Western Palearctic (WP), to test two key predictions of the master genes versus mass of genes views of speciation, and whether sex-linked loci are disproportionally involved without X/Z hemizygosity.First, we explored the strength of RI with increased divergence by comparing patterns of introgression across 41 pairs of naturally hybridizing lineages, 15 of which could be analyzed by geographic cline analyses. We expected a monotonous relationship under the mass of genes hypothesis, as RI results from the cumulative effects of multiple mutations arising with the genetic divergence of lineages. Under the master genes hypothesis, however, RI should not increase regularly but might suddenly arise at any moment along the continuum of divergence.Second, we examined the genomic landscape of introgression with locus-by-locus geographic cline analyses for a subset of nine transect-sampled hybrid zones targeted with restriction site–associated DNA–sequencing (RAD-seq) data. Under the master genes model, gene flow should first be drastically reduced at a few barrier loci and their surroundings (because of linked selection), while the rest of the genome still admixes more freely: Heterogeneous strengths of isolation among loci are thus expected when speciation starts. Under the mass of genes model, however, introgression should progressively decrease throughout the entire genome, as genetic divergence increases, until large parts entirely stop admixing: Heterogeneous levels of isolation are expected to appear later in the speciation process.Finally, we exploited whole-genome assemblies available for the genera Hyla and Rana, as well as knowledge of their sex determination systems (44, 45), to test for large X-effects across their respective hybrid zones. Since these frogs feature homomophic sex chromosomes, large X-effects would indicate that hemizygosity is not the main driver of sex-linked incompatibilities.