UC Berkeley

ABSTRACT

The Biogeography and Evolution of Symbiodinium in Giant Clams (Tridacnidae)

by

Michele Weber

Doctor of Philosophy in Integrative Biology

University of California, Berkeley

Professor Jere H. Lipps, Chair

Symbiodinium is a diverse lineage of dinoflagellates that forms photoendosymbioses with five phyla of marine invertebrates and protists. The symbiotic relationship provides hosts with a direct source of energetic resources and the dinoflagellates gain access to a safe environment where they can survive in high densities and easily access the raw materials for photosynthesis. In low nutrient, tropical waters, this relationship forms the base of the food web and the reef structure on which diverse communities have evolved.

Although many charismatic reef organisms show clear distribution patterns across their ranges, we are still exploring biogeography with respect to symbiosis. Previously biogeographic patterns in marine microbes were largely ignored; microbes were considered pandemic because lineages would not be limited to particular geographic regions as a result of dispersal and connectivity across oceans. More recently, advanced molecular techniques identified multiple scales of genetic variation in Symbiodinium that were not readily apparent from morphological analyses and encouraged investigation of finer scale questions about distributions and evolutionary patterns.

We now know that the genus Symbiodinium is diverse and that certain clades are found in unique regions and associate with particular hosts. Distributional data suggests that they were subjected to a selection mosaic and subgeneric clades diverged, geographically and functionally, on various scales but since the data is confined to certain hosts from certain regions, we know little about large-scale patterns. Some host specificity has been observed but since most hosts transmit their symbionts horizontally across generations and symbionts can be exchanged between host lineages, there is limited context for coevolutionary processes to refine particular pairings. I addressed the biogeographic patterns of Symbiodinium from giant clams on two different scales: regionally across their Indo West Pacific distribution and locally in the Northern Red Sea. Finally I analyzed the historical biogeography for the holobiont association between giant clams and Symbiodinium and inferred historical processes to explain the modern symbiont diversity patterns in a region of low host diversity.

In the first chapter I documented diversity of Symbiodinium for two species of giant clam, Tridacna maxima and Tridacna squamosa, across their Indo West Pacific distribution. At 25 localities across the Indo-Pacific from French Polynesia to the Red Sea, I collected small pieces of mantle tissue and recorded the depth and reef environment where each sample was collected. A complete distribution of Symbiodinium phylotypes in association with giant clams across their range provided a broad indication of what fraction of total Symbiodinium diversity associates with this host group. Compared to other host groups, Tridacna are specific hosts; of the hundreds of Symbiodinium genotypes documented in the literature and on GenBank, only ten distinct types live in giant clams and all had been reported from alternative hosts. Giant clams are crown group metazoans and each generation acquires new symbionts from the environment. While other alternative hosts, such as corals and foraminifera, can transfer symbionts between generations and occasionally evolve lineages of specific symbionts, I did not identify novel clam specific lineages. Giant clams host generalist Symbiodinium lineages that are readily available in the water column.

Although their distributional range is similar, T. squamosa and T. maxima did not host identical symbionts and I observed gradients in symbiont diversity. Symbionts were most diverse in the Central Indo West Pacific and diversity declined in the Pacific and Indian Oceans. While T. maxima was a generalist across more of its range and hosted diverse symbionts at most localities, T. squamosa was consistently specific for certain symbiont lineages and only in the Central Indo West Pacific did it host more diverse lineages of symbionts. T. squamosa lived on deeper reefs than T. maxima and the symbionts most often associated with T. squamosa exhibited a deeper range than some of the other lineages of Symbiodinium. However, T. maxima were also collected from deep reefs and they hosted different symbiont lineages. These data showed that multiple lineages of Symbiodinium are adapted to depth and they are partitioned between host species. The symbionts were also partitioned between different reef environments. Certain lineages were most common on patch reefs, others on fringing reefs and others from lagoon environments. Phylogenetic systematics indicated that the T. squamosa lineage is younger than the T. maxima lineage and my data on the distribution of Symbiodinium in the two lineages showed that it is also more symbiont specific. This evidence supported the hypothesis that T. squamosa is a less obligate host and can more rigorously select for high performing symbionts than T. maxima.

In the second chapter I focused on the lack of Symbiodinium diversity in T. maxima from the Red Sea. Only one phylotype of Symbiodinium was identified from T. maxima in the Red Sea and these sequences were not found in Tridacna samples from other regions. I proposed two hypotheses for the lack of diversity and concluded that multiple symbiont lineages had colonized the Red Sea but only a single lineage was successful and therefore, it replaced the other types and persisted. I compared the Red Sea phylotype to other Symbiodinium from alternative hosts in the Red Sea, the Mediterranean Sea and the West Indian Ocean, to show that the most closely related phylotypes exist along the coast of Kenya. I concluded that the Red Sea lineage originated in the West Indian Ocean and colonized the Red Sea via an alternative host that entered through the straits at Bab al Mandab, after the last glacial maximum, 12,000 years ago. Evidence of an endemic holobiont was evaluated with respect to the evolution of cooperation and transitional associations between partners on a geologic time scale. I suggested that the Red Sea phylotype is dominant because it was an infectious lineage. It easily colonized the new host population soon after the Red Sea reflooded, but the endemic holobiont may be transitional and as conditions stabilize, a more cooperative lineage will out-compete and replace the less efficient phylotype.

In the third chapter I addressed a diversity anomaly in the West Indian Ocean. The center of marine biodiversity is the Central Indo West Pacific, which includes Indonesia, the Philippines, Papua New Guinea and the northern Great Barrier Reef in Australia. I sampled more species of giant clam from Papua New Guinea and Australia and observed more symbiont lineages in association with those hosts than any other region within this study. In the West Indian Ocean I only observed two species of host, as was expected based on the marine biodiversity gradient. However, I also identified five lineages of Symbiodinium and while diversity in T. squamosa holobionts was lower in the West Indian Ocean, T. maxima holobionts were equally diverse in both regions. T. squamosa was a generalist in the Central Indo West Pacific and a specialist in the West Indian Ocean but T. maxima was a generalist in both regions. I proposed multiple hypotheses to account for these biogeographic patterns including geologic and oceanographic conditions, niche ecology and historical biogeographic patterns for the hosts. Historical biogeography of a holobiont system provides a new framework that includes the history of associations between partners as well as biogeographic patterns for each individual partner. In this case the history of the association between host and symbiont suggested what modern ecology could not explain. I showed that the holobiont range shifted more slowly than the host ranges and that the modern holobiont diversity in the West Indian Ocean is a legacy of Miocene diversity in that region.