Abstract:
Blooms of toxic species within the algal dinoflagellate species complex
Alexandrium tamarense may cause Paralytic Shellfish Poisoning, a significant and
growing environmental threat worldwide. However, blooms of closely related nontoxic
A. tamarense also occur, sometimes in close geographical proximity to toxic blooms.
This thesis explores the interactions between toxic and nontoxic blooms by examining
sexual crosses between each of five ribosomal clades within the A. tamarense complex
(termed Groups I-V). Several lines of evidence argue that these clades represent separate
species. Particular emphasis was given to interactions between toxic Group I and
nontoxic Group III species because they are among the most closely related A. tamarense
clades and because they share a natural range boundary in several parts of the world.
Interspecies hybridization appeared widespread between different clades and
between geographically dispersed isolates. However, subsequent germination studies of
hypnozygotes produced from combinations of Group I and Group III clones failed to
yield new vegetative cultures in multiple trials. The possibility that these hypnozygotes
were actually inbred (i.e. the result of pairs of only Group I or only Group III gametes)
was considered and rejected using a nested PCR assay that was developed to assess the
parentage of individual cysts. The assay was also suitable for analyzing cysts collected
from the field and was applied to individual cysts collected from Belfast Lough, an area
where both Group I and Group III blooms were known to occur. Two Group I/Group III
hybrids were detected in fourteen successful assays from the Belfast sample, showing
that hybridization does occur in nature.
These findings have several important implications. First, the failure of Group
I/Group III hybrids to produce new vegetative cultures serves as a proof that the A.
tamarense clades represent cryptic species because they are unable to produce genetic
intermediates. Second, the presence of hybrid cysts in Belfast Lough indicates ongoing
displacement of a nontoxic population by a toxic one (or vice versa) in that region. Third,
the inviability of toxic/nontoxic hybrids suggests a remediation strategy whereby the
recurrence of toxic A. tamarense blooms might be combated through the introduction of
nontoxic cells.
