Ecological adaptation of an F1 hybrid cross of carnivorous 1 and herbivorous Cyprinidae fishes 2

: 19 Background: Whether hybridization plays a positive or negative role in 20 speciation remains a controversial issue to date. Genetic factors have been 21 widely studied, but ecological factors also play an important role. Although 22 studies on the ecological adaptation of hybrids between different niche parents 23 have been widely reported, cases of extreme niche parental hybridization have 24 not been documented, which may show more ecological phenomena in the 25 fields of hybrid speciation and ecological species isolation. 26 Results: Taking Cyprinidae fish parents ( Schizothorax wangchiachii and 27 Percocypris pingi) with extreme ecological niches (herbivorous and 28 carnivorous) and their F1 hybrids as research objects, fish, shrimp, blood 29 worms and periphytic algae were selected as food correspond to four different 30 ecological niches. Morphologically, most external and skeletal traits in the F1 31 hybrids were balanced between the parents, but digestive traits were closer to 32 those of herbivorous parents. In terms of diet, the F1 hybrids weakly foraged 33 for parental food resources, but can more effectively forage for intermediate 34 food resources. In foraging abilities, the F1 hybrids showed low foraging 35 enthusiasm and abilities for parent resources, although the former was the 36 more important factor. Interestingly, the F1 hybrids showed high foraging 37 enthusiasm and success rates when they first foraged for fish, but then they 38 vomited fish debris as a result of mechanical difficulty in chewing rather than 39 taste, and the reason was a contradiction between the genetic behaviours and intermediate morphology. This behaviour was harmful and was persistent in 41 some individuals, representing a new mechanism in ecological species 42 isolation. However, the F1 hybrids have also shown evidence of new 43 ecological niche formation in favour of hybrid speciation by abandoning 44 foraging parent resources and focusing more on foraging intermediate foods. 45 Conclusions: (1) Low foraging enthusiasm is an important reason for the 46 fitness decrease of F1 hybrids to parent food. (2) The contradiction between 47 genetic behavior and intermediate traits is reported for the first time. (3) F1 48 hybrids may form an intermediate ecological niche between parents proved 49 experimentally. 50 51


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What role does hybridization play in speciation? Some researchers 56 thought that hybridization was an evolutionary dead end [1], because hybrids 57 were more often observed to be less healthy and ecologically adapted than 58 either parent species, and tended to be sterile [2][3][4][5][6][7]. However, others indicated 59 that hybridization can provide an important source of genetic variation on 60 which selection might act and that its adaptive role was more widespread  Of course, species foraging type is determined not only by traits but also 82 by foraging behaviour, with a significant correlation between the two [24]. 83 Foraging traits are often quantitative, and are therefore frequently additive 84 5 between parents in F1 hybrids. However, many unique behaviours develop 85 among species, and these unique parental genetic behaviours of F1 hybrids 86 may be codominant [25] or dominant [26] rather than additive. Therefore, 87 unique parental foraging behaviour is usually dominant rather than additive in 88 F1 hybrids, and the traits of the parents are often additive in the F1 hybrids, 89 which is obviously contradictory. 90 Due to the primitive evolutionary status of fish and in vitro fertilization, 91 more cases of hybridization are observed in fish than in higher vertebrates [12]. 92 Moreover, similar to plants, fish have many polyploids, especially Cyprinidae, 93 and this trait is often associated with the formation of allopolyploids by ancient 94 distant hybridization that can instantly create new species that are 95 reproductively isolated from their parents [27][28][29][30][31] indicating that hybridization 96 between parents with two different ecological niches will be more likely, and 97 genetically stable new species may form in Cyprinidae. 98 The species used in this study were cold-water Cyprinidae fishes from the 99 upper Yangtze River basin in the south-eastern Tibetan Plateau. quantify both external and skeletal characteristics, and age-one fishes (PP 129 (9.08±0.34 mm, 12.07±0.90 g), SW (9.23±0.14 mm, 13.03±1.50 g) and PS 130 (9.17±0.48 mm, 14.02±3.76 g)) were used to quantify digestive characteristics, 131 foraging and behavioural features.    where 1 represents body weight, and 2 represents chyme weight. 165 2.4 Hybrid vs P. pingi in foraging fish 166 We compared the foraging capacity of PP (n=15) and PS (n=18) for small 167 fishes (S. taeniatus) (Fig. 5a). Specific experimental methods are described in  interesting and important but is this behaviour persistent? We set up a feeding 213 experiment using small fish (Carassius auratus, 0.0748±0.0023 g (Fig. 6a)) for 214 nine days, and PS still had obvious vomiting behaviour after catching the small 215 C. auratus fishes (Fig. 6c)). For nine days, we fed not only fish, but we also fed  were the same as those in Section 2.6. However, the experiment lasted only 231 three days. We counted the average number of daily foraging (ANDF) and the 232 12 vomiting rate (VR) of the 7 experimental fishes used in Section 2.6 and this 233 experiment, which was equivalent to the former serving as a control group for 234 the latter, by the following formulas: Interestingly, we found a large amount of small fish debris in the PS 283 aquarium tank (Fig. 4e), while less debris was noted in the tanks with SW and 284 PP, suggesting that one of the reasons why fish intake by PS intake was low 285 was vomiting of fish. In the PS vs SW experiment, the FAT of PS was extremely significantly 289 higher (P<0.01) than that of SW (Fig. 5b), the AF was significantly lower 290 (P=0.02) than that of SW (Fig. 5c), and the FE was significantly lower (P=0.037) 291 than that of SW (Fig. 5e). In summary, PS showed low interest in foraging for 292 periphytic algae and had low foraging efficiency. those of PP; the FAT2 was extremely significantly higher (P<0.01) than that of 298 PP (Fig. 5f); and the AF of PS was extremely significantly lower (P<0.01) than 299 that of PP (Fig. 5g). In summary, PS showed greater interest in first foraging for 300 fish but had a high RVF, which caused it to be negative in later predation.   However, the VR of fish meat was significantly lower (P<0.01) than that of 314 small fish, suggesting that the vomiting behaviour was not caused by bad taste 315 but by chewing difficulty, which may be caused by pharyngeal tooth structure.

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Therefore, the details of the pharyngeal teeth were compared, and we found  and SW showed no significant difference (P≥0.05) on the third day (Fig. 8d).  only by the decline in PS foraging ability but, more importantly, by the decrease 364 in foraging activity (Fig. 5). The diet of a species depends not only on heredity  (Fig. 5). This result may be empirically negative.

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The behaviour of PS vomiting fish is one of the highlights of this study. caused not by bad taste but rather by chewing difficulty, which may be caused 377 by mechanical difficulties encountered due to the bones of small fish (Fig. 7).

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In the correlation analysis, we found no correlation between the structure of 379 any trait in PS and this behaviour (Supplementary Table S9). However, the 380 structure of pharyngeal teeth of PS was balanced between that of its parents, 381 but its puncture ability was not as good as that of PP; thus, PS may not reach 382 the threshold of normal chewing (Fig. 7)  predation. However, SP showed greater reductions than SW, which may be 396 due to the negativity brought by vomiting fish (Fig. 5 and Supplementary Fig. 3).

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The periphytic algae will not affect the foraging of PS for intermediate 398 ecological niche prey because PS is less interested in foraging on such algae 399 and has less interference ( Fig. 5 and Supplementary Fig. 3 decreases after the addition of suitable parental food resources, whereas that 408 of PS increases (Fig. 8) Ethics approval and consent to participate 426 The authors claim that none of the material in the paper has been 427 published or is under consideration for publication elsewhere. The submission 428 is original, and all authors are aware of the submission and agree to its 429 publication in Frontiers in Zoology. We declare that there is no conflict of 430 interests regarding the publication of this paper.

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Consent for publication 432 Written informed consent for publication was obtained from all participants.

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Availability of data and materials 434 The datasets used or analysed during the current study are available from 435 the corresponding author on reasonable request.

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Competing interests 437 All authors declare that they have no competing interests.