The evolution of hemocyanin genes in Tectipleura: a multitude of conserved introns in highly diverse gastropods

Abstract

Background Hemocyanin is the oxygen transporter of most molluscs. Since the oxygen affinity of hemocyanin is strongly temperature-dependent, this essential protein needs to be well-adapted to the environment. In Tectipleura, a very diverse group of gastropods with > 27,000 species living in all kinds of habitats, several hemocyanin genes have already been analyzed. Multiple independent duplications of this gene have been identified and may represent potential adaptations to different environments and lifestyles. The aim of this study is to further explore the evolution of these genes by analyzing their exon–intron architectures. Results We have reconstructed the gene architectures of ten hemocyanin genes from four Tectipleura species: Aplysia californica, Lymnaea stagnalis, Cornu aspersum and Helix pomatia. Their hemocyanin genes each contain 53 introns, significantly more than in the hemocyanin genes of Cephalopoda (9–11), Vetigastropoda (15) and Caenogastropoda (28–33). The gene structures of Tectipleura hemocyanins are identical in terms of intron number and location, with the exception of one out of two hemocyanin genes of L. stagnalis that comprises one additional intron. We found that gene structures that differ between molluscan lineages most probably evolved more recently through independent intron gains. Conclusions The strict conservation of the large number of introns in Tectipleura hemocyanin genes over 200 million years suggests the influence of a selective pressure on this gene structure. While we could not identify conserved sequence motifs within these introns, it may be simply the great number of introns that offers increased possibilities of gene regulation relative to hemocyanin genes with less introns and thus may have facilitated habitat shifts and speciation events. This hypothesis is supported by the relatively high number of introns within the hemocyanin genes of Pomacea canaliculata that has evolved independently of the Tectipleura. Pomacea canaliculata belongs to the Caenogastropoda, the sister group of Heterobranchia (that encompass Tectipleura) which is also very diverse and comprises species living in different habitats. Our findings provide a hint to some of the molecular mechanisms that may have supported the spectacular radiation of one of Metazoa’s most species rich groups.

The Evolution of Hemocyanin Genes in Caenogastropoda: Gene Duplications and Intron Accumulation in Highly Diverse Gastropods

“…Consequently, they represent one essential precondition for habitat shifts between strongly different environments (e.g., from sea to land or freshwater). Strong variability, as previously identified for hemocyanin genes in Tectipleura and now verified for five groups of Caenogastropoda, may accommodate these required adaptations (Schäfer et al 2018(Schäfer et al , 2021b.…”

“…Our findings reveal a strong diversity of hemocyanin genes of Caenogastropoda, including multiple independent gene duplications within different caenogastropod groups as well as a strong accumulation of FU-internal introns (21-57) within their genes. Since gene duplications and intron gains have also been discovered within hemocyanin genes of Tectipleura (Schäfer et al 2018(Schäfer et al , 2021b, they most likely represent general phenomena of hemocyanin gene evolution within Apogastropoda. This contrasts with the hemocyanin evolution of Vetigastropoda and may support the hypothesis that diversity in this oxygen transporter may increase adaptation.…”

“…The radiation that led to the high diversity of Apogastropoda involved a great number of adaptations, including the evolution of altered abilities for osmoregulation, novel respiratory organs (pneumostomes and lungs) and reproductive behavioral strategies (Vermeij and Dudley 2000;Vermeij and Wesselingh 2002). As we have proposed previously, high evolutionary rates within hemocyanin genes may represent molecular adaptations that have enabled multiple habitat shifts and species diversification within Apogastropoda (Schäfer et al 2018(Schäfer et al , 2021b. This hypothesis is developed from consideration of the strong temperature dependence of the oxygen affinity of hemocyanins (Brix et al 1989;Burnett et al 1988;Mangum 1990).…”

“…HpH in Table 2 and Fig. 4; Schäfer et al 2021b), with the exception of MtH 400 of M. tuberculata. Hemocyanin genes of this cerithioid encompass the greatest number of internal introns that have been identified thus far.…”

“…In addition to multiple independent gene duplications, our results on exon-intron architectures also reveal a high rate of evolutionary changes in hemocyanin genes in Caenogastropoda. We have previously reported that hemocyanin genes of Tectipleura encompass a significantly larger number of FU-internal introns (on average 5.6 introns per FU; Schäfer et al 2021b) than hemocyanin genes of Vetigastropoda or Cephalopoda (≤ 0.8 introns per FU). This study also identified a larger number of internal introns in hemocyanin genes of Caenogastropoda (2.4-6.5 introns per FU), the other branch of Apogastropoda (Fig.…”