Metagenomic analysis reveals hidden links between gut microbes and habitat adaptation among cave and surface dwelling Sinocyclocheilus species.

Intestinal microbes are closely related to vital host functions such as digestion and nutrient absorption, which play important roles in enhancing host adaptability. As a natural "laboratory", caves provide an outstanding model for understanding the significance of gut microbes and feeding habits in the habitat adaptability of hosts. However, research on the relationship between gut microbes, feeding habits, and the adaptability of troglobites remains insufficient. In this study, we compared the characteristics of the intestinal microbes of Sinocyclocheilus cavefish and surface fish and further established the relationship between intestinal and habitat microbes. Furthermore, we conducted environmental DNA (eDNA) (metabarcoding) analysis of environmental samples to clarify the composition of potential food resources in the habitats of the Sinocyclocheilus cavefish and surface fish. Results showed that the structure of the Sinocyclocheilus gut microbes was more related to ecological type (habitat type) than phylogenetic relationships. While horizontal transfer of habitat microbes was a source of gut microbes, hosts also showed strong selection for inherent microbes as dominant microorganisms. Differences in the composition and structure of gut microbes, especially dominant microbes, may enhance the adaptability of the two Sinocyclocheilus fish types from the perspectives of food intake, nutrient utilization, and harmful substance metabolism, suggesting that food resources, predation patterns, intestinal flora, digestive and absorptive capacity, and feeding habits and preferences are linked to habitat adaptability. These results should facilitate our understanding of the significance of fish gut microbes to habitat adaptation and provide a new perspective for studying the adaptive mechanisms of cavefish.

Microscopic and Submicroscopic Gradient Variation of Olfactory Systems among Six Sinocyclocheilus Species Living in Different Environments.

The fish genus Sinocyclocheilus contains many different species that inhabit diverse natural environments, such as surface water layer, cave, or intermediate. As a result of these different habitats there are some differences in their sensory systems. Microscopic and submicroscopic structures of olfactory systems in six representative species of Sinocyclocheilus were studied, including one surface-dwelling species (S. grahami), two intermediate species (S. jii and S. macrophthalmus) and three cave-dwelling species (S. brevibarbatus, S. anshuiensis, and S. tianlinensis). Due to adaptive evolution under extreme environmental conditions, cave-dwelling species have more developed olfactory systems. We observed that, compared with surface-dwelling species, the olfactory sac of the cave-dwelling Sinocyclocheilus species has the following characteristics: higher density of cilia, greater length of sensory cilia, many other special structures (micro-ridge, olfactory islet, rod cilia). These results reveal different levels of olfactory system development, consistent with the view that that cave-dwelling species have more developed olfactory systems than intermediate and surface-dwelling species.

Testing the validity of two putative sympatric species from Sinocyclocheilus (Cypriniformes: Cyprinidae) based on mitochondrial cytochrome b sequences.

There are over 60 species within the freshwater fish genus Sinocyclocheilus (Cypriniformes: Cyprinidae) distributed throughout the Yunnan-Guizhou Plateau and its surrounding areas in China. In recent years, the increasing number of new species described has raised some controversy about the validity of several species within this genus, notably the putative sympatric species pair S. qiubeiensis and S. jiuchengensis. To test the validity of S. qiubeiensis and S. jiuchengensis, we analyzed the complete sequences of the mitochondrial cytochrome b (CYTB) gene of 20 identified species and one outgroup species. Phylogenetic relationships were reconstructed using CYTB with maximum likelihood (ML) and Bayesian inference (BI) methods. Our phylogenetic results showed that all individuals of S. qiubeiensis and S. jiuchengensis clustered in one clade with strong support. In addition, the genetic distance between the two species was 0.11%, within the range observed at the intraspecific level. The most recent common ancestor of S. qiubeiensis and S. jiuchengensis dated back to 0.13 million years ago, indicating little time for complete speciation to have occurred. These results clearly support the viewpoint that S. jiuchengensis is a synonym of S. qiubeiensis.

Biological butanol production from microalgae-based biodiesel residues by Clostridium acetobutylicum.

This study conducted batch experiments to evaluate the potential of butanol production from microalgae biodiesel residues by Clostridium acetobutylicum. The results indicated that with 90 g/L of glucose as the sole substrate the highest butanol yield of 0.2 g/g-glucose was found, but the addition of butyrate significantly enhanced the butanol yield. The highest butanol yield of 0.4 g/g-glucose was found with 60 g/L of glucose and 18 g/L of butyrate. Using microalgae biodiesel residues as substrate, C. acetobutylicum produced 3.86 g/L of butanol and achieved butanol yield of 0.13 g/g-carbohydrate via ABE fermentation, but the results indicated that approximately one third of carbohydrate was not utilized by C. acetobutylicum. Biological butanol production from microalgae biodiesel residues can be possible, but further research on fermentation strategies are required to improve production yield.

Origin, genetic diversity, and population structure of Chinese domestic sheep.

To characterize the origin, genetic diversity, and phylogeographic structure of Chinese domestic sheep, we here analyzed a 531-bp fragment of mtDNA control region of 449 Chinese autochthonous sheep from 19 breeds/populations from 13 geographic regions, together with previously reported 44 sequences from Chinese indigenous sheep. Phylogenetic analysis showed that all three previously defined lineages A, B, and C were found in all sampled Chinese sheep populations, except for the absence of lineage C in four populations. Network profiles revealed that the lineages B and C displayed a star-like phylogeny with the founder haplotype in the centre, and that two star-like subclades with two founder haplotypes were identified in lineage A. The pattern of genetic variation in lineage A, together with the divergence time between the two central founder haplotypes suggested that two independent domestication events have occurred in sheep lineage A. Considerable mitochondrial diversity was observed in Chinese sheep. Weak structuring was observed either among Chinese indigenous sheep populations or between Asian and European sheep and this can be attributable to long-term strong gene flow induced by historical human movements. The high levels of intra-population diversity in Chinese sheep and the weak phylogeographic structuring indicated three geographically independent domestication events have occurred and the domestication place was not only confined to the Near East, but also occurred in other regions.

Mitochondrial diversity and phylogeographic structure of Chinese domestic goats.

China has numerous native domestic goat breeds, but so far there has been no extensive study on genetic diversity, population demographic history, and origin of Chinese goats. Here, we examined the genetic diversity and phylogeographic structure of Chinese domestic goats by determining a 481-bp fragment of the first hypervariable region of mitochondrial DNA (mtDNA) control region from 368 individuals representing 18 indigenous breeds. Phylogenetic analyses revealed that there were four mtDNA lineages (A-D) identified in Chinese goats, in which lineage A was predominant, lineage B was moderate, and lineages C and D were at low frequency. These results further support the multiple maternal origins of domestic goats. The pattern of genetic variation in goat mtDNA sequences indicated that the two larger lineages A and B had undergone population expansion events. In a combined analysis of previously reported sequences and our sequences belonging to lineage B, we detected two subclades, in which one was unique to eastern Asia and another was shared between eastern and southern Asia. A larger genetic variation in eastern Asia than southern Asia and the pattern of phylogeographic variation in lineage B suggest that at least one subclade of lineage B originated from eastern Asia. There was no significant geographical structuring in Chinese goat populations, which suggested that there existed strong gene flow among goat populations caused by extensive transportation of goats in history.

Molecular phylogeny of Sinocyclocheilus (Cypriniformes: Cyprinidae) inferred from mitochondrial DNA sequences.

More than 10 species within the freshwater fish genus Sinoncyclocheilus adapt to caves and show different degrees of degeneration of eyes and pigmentation. Therefore, this genus can be useful for studying evolutionary developmental mechanisms, role of natural selection and adaptation in cave animals. To better understand these processes, it is indispensable to have background knowledge about phylogenetic relationships of surface and cave species within this genus. To investigate phylogenetic relationships among species within this genus, we determined nucleotide sequences of complete mitochondrial cytochrome b gene (1140 bp) and partial ND4 gene (1032 bp) of 31 recognized ingroup species and one outgroup species Barbodes laticeps. Phylogenetic trees were reconstructed using maximum parsimony, Bayesian, and maximum likelihood analyses. Our phylogenetic results showed that all species except for two surface species S. jii and S. macrolepis clustered as five major monophyletic clades (I, II, III, IV, and V) with strong supports. S. jii was the most basal species in all analyses, but the position of S. macrolepis was not resolved. The cave species were polyphyletic and occurred in these five major clades. Our results indicate that adaptation to cave environments has occurred multiple times during the evolutionary history of Sinocyclocheilus. The branching orders among the clades I, II, III, and IV were not resolved, and this might be due to early rapid radiation in Sinocyclocheilus. All species distributed in Yunnan except for S. rhinocerous and S. hyalinus formed a strongly supported monophyletic group (clade V), probably reflecting their common origins. This result suggested that the diversification of Sinocyclocheilus in Yunnan may correlate with the uplifting of Yunnan Plateau.