Resource Polymorphism In Schizopygopsis Thermalis From A Headwater Lake In The Salween River In The Tibetan Plateau

Resource polymorphism is a phenomenon whereby a single species exhibits two or more morphs?morphotypes?showing differences in morphology,behaviour,coloration,or life history characteristics?e.g.,growth characters?.These differences may be considered as an adaptation to habitat heterogeneity through the differentiation of feeding biology and habitat utilization.Resource polymorphism often shows the utilization of different resources and the existence of different degrees of reproductive isolation,which represents an important intermediate stage in the process of speciation.Meanwhile,resource polymorphism plays a critical role in the construction of a speciation model with habitat specialization and reproductive isolation as prerequisites.Therefore,species and natural systems with resource polymorphism provide unique research opportunities for understanding the processes and mechanisms of ecological speciation and exploring the driving factors of diversity.It also provides important practical support for the development of ecological speciation model.The lake in Tibetan Plateau is characterized by diverse lacustrine environments,separated and obviously different habitat and shows minimal interspecific competition due to scarce species,and possesses the basement of morphological and ecological differentiation.We aimed to analyze:1)the morphological polymorphism in Schizopygopsis thermalis in Lake Amdo Tsonak Co,describe and distinguish different morph morphological characters by a combination of morphometric and traditional linear measures;2)analyze the biological characters and difference?e.g.,feeding habit,growth and reproductive traits?of the two morphs;3)to define whether genetic differentiation occurs between morphs;4)finally,to explore the relationship between morphological differentiation and habitat utilization,and to elucidate the potential ecological mechanism of resource polymorphism.Our main results were as follows:?1?All samples of S.thermalis were identified two group via UPGMA cluster analysis:planktivorous morph and benthivorous morph.DFA showed a highly correctly allocated,a posteriori jackknife cross-validation showed an average at98.7%.Based on the PCA and reconstructed body shape results,the samples of the planktivorous morph had a equal length of upper and lower jaw,a longer head length,more robust head shape and a longer upper jaw,lower jaw and snout length than those of the benthivorous morph.Sexes had no significant effect on the classification of morphologic groups.The infection rate of parasites in both of morphs was low?4.1%and 11.3%,respectively?and parasitism also did not affect classification of morphologic groups.The two morphs?planktivorous and benthivorous?significantly differed in head characters.Planktivorous individuals possessed a terminal mouth with a slightly horny edge?width of horny edge:<0.02 cm?or lacked a horny edge on the lower jaw and had a highly developed mucus cavity in the cheek and chin.The benthivorous individuals were characterized by an inferior or sub-inferior mouth with a sharpened horny edge on the lower jaw?width of horny edge:0.14-0.42 cm?and the lack of a mucus cavity or only a small mucus cavity in the cheek and chin.Pharyngeal teeth in one or two rows were observed in both morphs,though the percentage of individuals with a single row of pharyngeal teeth was higher in the benthivorous morph than in the planktivorous morph.The numbers and lengths of gill rakers did not had significant between morphs.?2?Although both of two morphs belongings to a typical omnivorous fish,they had a low dietary overlap in food resources(C_xy=0.42).The group of planktivorous morph predominately fed on plankton,small fishes?like Triyplophysa?and hydrophilic insects while benthivorous morph mainly fed on periphytic algae,small fishes and benthos.?3?Both morphs had a significant difference in growth:the planktivorous morph exhibits a larger asymptotic standard length?L_??,lower growth rate?k?and higher growth performance index???than the benthivorous morph.?4?Phylogenetic relationship was reconstructed by the two mitochondrial gene fragments?Cyt b+D-Loop?,it was found that all the individuals of the S.thermalis clustered into a genetically mixed population,which did not support the monophyletic status of the two morphs.AMOVA revealed that most of the genetic variation was found within morphs and little genetic variation was present between the two morphs.The genetic differentiation between two morphs was weak.The above results indicated that there were two morphs in the distribution of the S.thermalis in Lake Amdo Tsonak Co and there were significant differences in feeding habits and growth between the two morphs.The weak but significant genetic differences between the two morphs indicate that the two morphs are still in the initial stage of species differentiation.Lake Amdo Tsonak Co,which is characterized by a diverse lacustrine environments,separated and obviously different habitat and shows minimal interspecific competition,provided environment basement of morphological polymorphism for S.thermalis;while the freshwater environment made it possible to isolate the breeding sites of the two morphs,which further consolidated and promoted the differentiation of the two morphs.The study of the resource polymorphism in S.thermalis might provide a unique research opportunity to understand the process and mechanism of speciation in the schizothoracine fishes,and explore the driving factors of polymorphism.It also provides important practical support for the development of ecological speciation model of schizothoracine fishes.