Background Postzygote isolation is an important part of species isolation, especially for fish, and it can be divided into two aspects: genetic isolation and ecological isolation. With the increase in parental genetic distance, the intensity of genetic isolation between them also increases. Will the increase in parental ecological niche differences also lead to the increase in ecological isolation intensity between them? This question is difficult to answer based on the current literature due to the lack of hybridization cases of contrasting ecological niche parents. Results Cyprinid fish parents (Schizothorax wangchiachii and Percocypris pingi) with contrasting ecological niches (herbivorous and carnivorous) and their F1 hybrids were used as research objects. Fish and periphytic algae were selected as food corresponding to different parental resources. The foraging-related traits of these hybrids are generally the same between parents; however, the intermediate foraging traits of hybrids did not result in intermediate foraging performance for parental resources, and these hybrids could hardly forage for parental resources. The poor foraging performance of these hybrids for parental resources was caused not only by the decline in the foraging ability of these hybrids but, more importantly, by the decrease in foraging activity. Interestingly, these hybrids initially showed a high interest in foraging small fishes; however, after the first successful capture, these hybrids had difficulty ingesting fish and spit them out, which led to the subsequent decrease in foraging activity. We designed a series of experiments to explore the mechanism of the fish spitting of these hybrids, excluding the taste and the size of prey, and found that the decrease in their pharyngeal tooth puncture ability may be the reason. Conclusions This study was the first to demonstrate that these parents with contrasting ecological niches will produce great postzygotic ecological isolation for parental resources. The poor foraging performance of these hybrids for parental resources is mainly due to the decrease in foraging activity. Interestingly, these hybrids have obvious fish-spitting behaviour, which is a typical example of the incompatibility between intermediate traits and genetic behaviors.

In the process of sympatric species differentiation, which feeding traits are most differentiated? The complete feeding process of capture, ingestion, and digestion requires certain external, as well as internal traits. Therefore, comprehensive comparisons are needed to study fish feeding habits. This study systematically investigated the adaptive radiation and parallel evolution of Schizothorax fishes, and quantified the external, frame, pharyngeal bone, and digestive traits of the systematically differentiated omnivorous Schizothorax davidi and herbivorous S. wangchiachii. A member of their sister genus, the carnivorous Percocypris pingi, was used as an outgroup control to explore the feeding differentiation of sympatric Schizothorax fishes. The results showed that these species had highly differentiated external traits, while the internal traits (pharyngeal bone and digestive traits) showed low differentiation, indicating that predatory traits diverged first in the differentiation of sympatric Schizothorax fishes, while chewing and digestive traits were slower to diverge. Furthermore, we screened 16 continuous feeding traits, and the quantified values of these traits progressively increased or decreased with increasing nutritional level of food. These findings provide a reference for the selection of quantitative indicators in future studies of sympatric species differentiation.

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Background: Postzygote isolation is an important part of species isolation, especially for fish, and it can be divided into two aspects: genetic isolation and ecological isolation. With the increase in parental genetic distance, the intensity of genetic isolation between them also increases. Will the increase in parental ecological niche differences also lead to the increase in ecological isolation intensity between them? This question is difficult to answer based on the current literature due to the lack of hybridization cases of extreme ecological niche parents.Results: Cyprinidae fish parents (Schizothorax wangchiachii and Percocypris pingi) with extreme ecological niches (herbivorous and carnivorous) and their F1 hybrids were used as research objects. Fish and periphytic algae were selected as food corresponding to different parental resources. The foraging-related traits of these hybrids are generally the same between parents; however, the intermediate foraging traits of hybrids did not result in intermediate foraging performance for parental resources, and these hybrids could hardly forage for parental resources. The poor foraging performance of these hybrids for parental resources was caused not only by the decline in the foraging ability of these hybrids but, more importantly, by the decrease in foraging activity. Interestingly, these hybrids initially showed a high interest in foraging small fishes; however, after the first successful capture, these hybrids had difficulty ingesting fish and spit them out, which led to the subsequent decrease in foraging activity. We designed a series of experiments to explore the mechanism of the fish spitting of these hybrids, excluding the taste and the size of prey, and found that the decrease in their pharyngeal tooth puncture ability may be the reason. Conclusions: This study was the first to demonstrate that these parents with extreme niche differences will produce extreme postzygotic ecological isolation for parental resources. The poor foraging performance of these hybrids for parental resources is mainly due to the decrease in foraging activity. Interestingly, these hybrids have obvious fish-spitting behaviour, which is a typical example of the contradiction between intermediate traits and parental resources.

Background Whether hybridization plays a positive or negative role in speciation remains a controversial issue to date. Genetic factors have been widely studied, but ecological factors also play an important role. Although studies on the ecological adaptation of hybrids between different niche parents have been widely reported, cases of extreme niche parental hybridization have not been documented, which may show more ecological phenomena in the fields of hybrid speciation and ecological species isolation. Results Taking Cyprinidae fish parents (Schizothorax wangchiachii and Percocypris pingi) with extreme ecological niches (herbivorous and carnivorous) and their F1 hybrids as research objects, fish, shrimp, blood worms and periphytic algae were selected as food correspond to four different ecological niches. Morphologically, most external and skeletal traits in the F1 hybrids were balanced between the parents, but digestive traits were closer to those of herbivorous parents. In terms of diet, the F1 hybrids weakly foraged for parental food resources, but can more effectively forage for intermediate food resources. In foraging abilities, the F1 hybrids showed low foraging enthusiasm and abilities for parent resources, although the former was the more important factor. Interestingly, the F1 hybrids showed high foraging enthusiasm and success rates when they first foraged for fish, but then they vomited fish debris as a result of mechanical difficulty in chewing rather than taste, and the reason was a contradiction between the genetic behaviours and intermediate morphology. This behaviour was harmful and was persistent in some individuals, representing a new mechanism in ecological species isolation. However, the F1 hybrids have also shown evidence of new ecological niche formation in favour of hybrid speciation by abandoning foraging parent resources and focusing more on foraging intermediate foods. Conclusions (1) Low foraging enthusiasm is an important reason for the fitness decrease of F1 hybrids to parent food. (2) The contradiction between genetic behavior and intermediate traits is reported for the first time. (3) F1 hybrids may form an intermediate ecological niche between parents proved experimentally.