5:15 PM - 7:15 PM
[BCG07-P06] Correspondence between genotype and symbiotic algae of the planktonic foraminifera Globigerinella siphonifera
Keywords:Planktonic foraminifera, Photosymbiosis, Cryptic species, Molecular phylogenetic analysis
Planktonic foraminifera are unicellular marine eukaryotes with calcite shells, which are valuable for biostratigraphy and paleoenvironmental reconstruction. Among modern planktonic foraminifera, symbiotic algae have been confirmed in 17 species, typically with one symbiont species associated with each host species.
On the other hand, Globigerinella siphonifera has been known to have two symbiont species, Chrysochromulina and Pelagomonas, corresponding to different host genotypes. Currently, four morphospecies within the genus Globigerinella have been described: G. adamsi, G. siphonifera, G. calida, and G. radians. Identifying species other than G. adamsi morphologically can be challenging, and they have often been treated as variations in G. siphonifera (hereafter called G. siphonifera plexus). Within this plexus, 12 genotypes have been identified (Weiner et al., 2014). Although Weiner et al. (2015) summarized the correspondence between the three morphospecies and the 12 genotypes, we will treat them all here as genotypes of the G. siphonifera plexus. It is now established that there are 12 genotypes within the G. siphonifera plexus, and that there are at least two different symbiotic algae. However, the specific symbiont species associated with each host genotype remains unclear. Therefore, this study aims to investigate the correspondence between the host genotypes and their symbiotic algae, and to elucidate their phylogenetic relationships.
The samples used in this study were collected during cruises on the Hakuho-maru: KH-17-4 (western North Pacific), KH-19-6 (eastern South Pacific), and from offshore Manazuru, Sagami Bay. A total of 101 individuals were analyzed. We targeted three gene regions that can resolve foraminiferal genotypes and symbiont species: the 18S rRNA gene of foraminifera (partial, 1080 bp), the 18S rRNA gene of eukaryotic algae (partial, 758 bp), and the rbcL gene of algal chloroplasts (partial, 502 bp). The obtained sequences were aligned and used to reconstruct phylogenetic trees using the maximum likelihood method.
In the G. siphonifera plexus lineage, we identified 11 out of the 12 known genotypes (excluding Type IIIb). The symbiotic algae were classified as either haptophyte (single species Chrysochromulina andersonii) or pelagophytes (Pelagomonas calceolata or Pelagophyceae sp.) based on the partial region of the rRNA gene. The lineages of pelagophytes inferred from the rbcL gene were subdivided into two clades (Pelagophyceae 1 and 2).
The correspondence between the G. siphonifera plexus genotypes and symbiotic algae is as follows: Type I has haptophyte (C. andersonii), Type II has pelagophytes ( P. calceolata, Pelagophyceae1, 2), while Type III has no symbiotic algae. It is likely that the symbiotic relationships were established independently for Type I and Type II, according to the most parsimonious scenario. Our results also revealed that Type IIa and Type IIb were associated with closely related but distinct pelagophytes: P. calceolata and Pelagophyceae sp., respectively. However, there were exceptions where Type IIa individuals also had Pelagophyceae sp.. It can be hypothesized that Pelagophyceae sp. was acquired in the common ancestor of Type IIa and IIb, followed by a symbiont switch from Pelagophyceae sp. to P. calceolata in Type IIa after the divergence of the host genotypes. Further exploration of the biogeography of these genotypes is required to better understand the specificity of their symbiotic partnerships and the evolution of these relationships.
On the other hand, Globigerinella siphonifera has been known to have two symbiont species, Chrysochromulina and Pelagomonas, corresponding to different host genotypes. Currently, four morphospecies within the genus Globigerinella have been described: G. adamsi, G. siphonifera, G. calida, and G. radians. Identifying species other than G. adamsi morphologically can be challenging, and they have often been treated as variations in G. siphonifera (hereafter called G. siphonifera plexus). Within this plexus, 12 genotypes have been identified (Weiner et al., 2014). Although Weiner et al. (2015) summarized the correspondence between the three morphospecies and the 12 genotypes, we will treat them all here as genotypes of the G. siphonifera plexus. It is now established that there are 12 genotypes within the G. siphonifera plexus, and that there are at least two different symbiotic algae. However, the specific symbiont species associated with each host genotype remains unclear. Therefore, this study aims to investigate the correspondence between the host genotypes and their symbiotic algae, and to elucidate their phylogenetic relationships.
The samples used in this study were collected during cruises on the Hakuho-maru: KH-17-4 (western North Pacific), KH-19-6 (eastern South Pacific), and from offshore Manazuru, Sagami Bay. A total of 101 individuals were analyzed. We targeted three gene regions that can resolve foraminiferal genotypes and symbiont species: the 18S rRNA gene of foraminifera (partial, 1080 bp), the 18S rRNA gene of eukaryotic algae (partial, 758 bp), and the rbcL gene of algal chloroplasts (partial, 502 bp). The obtained sequences were aligned and used to reconstruct phylogenetic trees using the maximum likelihood method.
In the G. siphonifera plexus lineage, we identified 11 out of the 12 known genotypes (excluding Type IIIb). The symbiotic algae were classified as either haptophyte (single species Chrysochromulina andersonii) or pelagophytes (Pelagomonas calceolata or Pelagophyceae sp.) based on the partial region of the rRNA gene. The lineages of pelagophytes inferred from the rbcL gene were subdivided into two clades (Pelagophyceae 1 and 2).
The correspondence between the G. siphonifera plexus genotypes and symbiotic algae is as follows: Type I has haptophyte (C. andersonii), Type II has pelagophytes ( P. calceolata, Pelagophyceae1, 2), while Type III has no symbiotic algae. It is likely that the symbiotic relationships were established independently for Type I and Type II, according to the most parsimonious scenario. Our results also revealed that Type IIa and Type IIb were associated with closely related but distinct pelagophytes: P. calceolata and Pelagophyceae sp., respectively. However, there were exceptions where Type IIa individuals also had Pelagophyceae sp.. It can be hypothesized that Pelagophyceae sp. was acquired in the common ancestor of Type IIa and IIb, followed by a symbiont switch from Pelagophyceae sp. to P. calceolata in Type IIa after the divergence of the host genotypes. Further exploration of the biogeography of these genotypes is required to better understand the specificity of their symbiotic partnerships and the evolution of these relationships.