Abstract
Hemocyanin is the oxygen transport protein of most molluscs and represents an important physiological factor that has to be well-adapted to their environments because of the strong influences of abiotic factors on its oxygen affinity. Multiple independent gene duplications and intron gains have been reported for hemocyanin genes of Tectipleura (Heterobranchia) and the caenogastropod species Pomacea canaliculata, which contrast with the uniform gene architectures of hemocyanins in Vetigastropoda. The goal of this study was to analyze hemocyanin gene evolution within the diverse group of Caenogastropoda in more detail. Our findings reveal multiple gene duplications and intron gains and imply that these represent general features of Apogastropoda hemocyanins. Whereas hemocyanin exon–intron structures are identical within different Tectipleura lineages, they differ strongly within Caenogastropoda among phylogenetic groups as well as between paralogous hemocyanin genes of the same species. Thus, intron accumulation took place more gradually within Caenogastropoda but finally led to a similar consequence, namely, a multitude of introns. Since both phenomena occurred independently within Heterobranchia and Caenogastropoda, the results support the hypothesis that introns may contribute to adaptive radiation by offering new opportunities for genetic variability (multiple paralogs that may evolve differently) and regulation (multiple introns). Our study indicates that adaptation of hemocyanin genes may be one of several factors that contributed to the evolution of the large diversity of Apogastropoda. While questions remain, this hypothesis is presented as a starting point for the further study of hemocyanin genes and possible correlations between hemocyanin diversity and adaptive radiation.
Highlights
Mollusca is the second largest animal phylum and includes over 82,000 extant species
To reveal relationships between different hemocyanin paralogs and to identify hemocyanin gene duplication events within Caenogastropoda, we inferred the phylogeny of these proteins, including eleven hemocyanin sequences of over 10,000 nucleotides each from species that belong to four different main groups of Caenogastropoda
We revised the hemocyanin sequences of the Ampullariida species P. canaliculata and newly assembled the cDNA of the hemocyanins of L. saxatilis, which belongs to the asiphonate Hypsogastropoda (Fig. 2)
Summary
Mollusca is the second largest animal phylum and includes over 82,000 extant species (for numbers cf. WoRMS Editorial Board 2020). The great diversity of this phylum is represented best by the two large gastropod groups Heterobranchia and Caenogastropoda which together form the clade Apogastropoda. They comprise over 64,000 species living in various habitats including the sea, fresh waters. Another very important factor of gastropod respiration that has to be adapted to environmental conditions is the oxygen transporter hemocyanin.
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