ENGLISH ABSTRACT: South Africa is known for its high marine invertebrate diversity but the
evolutionary histories of these species are largely unknown. The present study
contributes to the growing body of phylogeographic studies of southern African coastal
species. The main aim is to better understand the colonization and diversification of
South African barnacles.
To investigate the phylogeographic pattern in the southern African volcano
barnacle, Tetraclita serrata, 410 individuals from 20 sampling localities were analyzed.
In addition, to understand the colonization and diversification patterns of South African
barnacle species, nine taxa were included in a molecular phylogeny derived from the
nuclear gene, 18S rRNA. With only a limited number of 18S sequences available on
GenBank, a separate phylogenetic tree, for the mitochondrial gene, was constructed to
determine whether the genus Tetraclita is monophyletic.
Restricted gene flow in some geographical areas was hypothesized for T. serrata
based on oceanic circulation patterns; known biogeographic regions; and features such
as the Agulhas Bank, which has shaped the population genetic structure of several other
South African marine organisms. The population genetic structure was investigated
using the mitochondrial cytochrome oxidase subunit 1 (CO1) and nuclear internal
transcribed spacer 1 (ITS1) genes. Phylogenetic and population genetic analyses of the
CO1 gene revealed two distinct genetic clades with overlapping distributions. The
nuclear ITS1 data performed on a subset of individuals drawn from both mtDNA clades
revealed a single lineage. The pattern observed can be ascribed to a historical event that
may have been responsible for the formation of allopatric lineages that have since come
into secondary contact. On the other hand, the pattern observed may be as result of
incomplete sorting of nDNA alleles, in which case, given that the two mtDNA clades are
not geographically isolated, could be explained by selective pressures acting on the
species due to ecological constraints. No clear phylogeographic structure was found
within each of these clades and the direction of gene flow of T. serrata individuals can be
linked to the oceanographic features found along the South African coast. In contrast to
most other South African marine species studied to date, the haplotype network,
mismatch distributions and time since expansion suggest that the effective population size of T. serrata was not severely affected by the Last Glacial Maximum. It is proposed
that further investigations into the phylogeography of coastal marine species,
particularly obligatory sessile species such as barnacles, are required to determine
whether the patterns observed in T. serrata is a rare history, or not.
Neighbour-joining, maximum parsimony and Bayesian analyses on the CO1 gene
provide evidence for the monophyly of the genus Tetraclita; however, Tetraclita species
found in South Africa do not share a common ancestry suggesting multiple colonization
events. This study has also accidently led to the discovery of an introduced species,
Balanus perforatus, native to Europe, and I discuss the potential of the alien becoming
invasive on the east coast of South Africa where it was found.
AFRIKAANSE OPSOMMING: Suid-Afrika is bekend vir sy hoë mariene ongewerwelde diversiteit, maar die
evolusionêre geskiedenis van hierdie spesies is grootliks onbekend. Hierdie studie is
gedoen om by te dra tot die filografiese studies van suider Afrikaanse kus spesies en om
die kolonisasie en diversifikasie van die Suid-Afrikaanse eendmossels beter te verstaan.
Om die genetiese struktuur van die bevolkings van die vulkaan eendmossel,
Tetraclita serrata, wat langs die Suid-Afrikaanse kuslyn voorkom, te bestudeer, was 410
individue van 20 lokaliteite ontleed. Daarbenewens, om die kolonisasie en diversifikasie
van Suid-Afrikaanse eendmossels te verstaan, was nege spesies ingesluit in 'n
molekulêre filogenie wat gebaseer is op die kern geen, 18s rRNA. Met slegs 'n beperkte
aantal 18s DNS volgordes beskikbaar op GenBank,is 'n aparte filogenetiese boom, vir die
mitochondriale geen COI, gekonstrueer om te bepaal of die genus Tetraclita monofileties
is.
Beperkte geen-vloei in sommige geografiese gebiede was verwag vir T. serrrata
gebaseer op oseaniese sirkulasiepatrone; bekende biogeografiese streke, en kenmerke
soos die Agulhas Bank, wat die filogeografiese struktuur van verskeie ander Suid-
Afrikaanse mariene organismes beïnvloed het. Die genetiese struktuurvan die
bevolkings is geondersoek met behulp van die mitochondriale sitochroom oksidase
subeenheid 1 (COI) en kern interne getranskripeerde spasieërder 1 (ITS1) gene. Geen
duidelike bevolkings genetiese struktuur is gevind nie en die rigting van geenvloei van
T. serrata individue kan gekoppel word aan die oseanografiese kenmerke wat langs die
Suid-Afrikaanse kus voorkom. Filogenetiese en bevolking genetiese ontleding van die
COI geen openbaar twee afsonderlike klades maar met oorvleuelende geografiese
verspruidings. Die ITS1 data-analise wat uitgevoer was op 'n subset van individue wys
op 'n enkele spesie. Die waargenome patroon dui op 'n belangrike historiese verskil
tussen die twee klades. 'n Geskiedkundige gebeurtenis was dalk verantwoordelik vir die
vorming van twee evolusionêre lyne wat sederdien sekondêre kontak het. Aan die ander
kant, kan die patroon waargeneem word as gevolg van die onvolledige sortering van
nDNA allele, in welke geval, gegee dat die twee mtDNA clades nie geografies geïsoleer is
nie, dit verduidelik kan word deur selektiewe druk wat op die spesie was as gevolg van
fisiologiese of ekologiese beperkings. Die statistiese parsimonie netwerk, ongelyksoortige verspreidings en tyd sedert die bevolkingsuitbreiding dui daarop dat T.
serrata die laaste ysagtige maksimum tydperk oortleef het. Tot op hede het geen Suid-
Afrikaanse mariene spesies so 'n patroon gewys nie. So, verdere ondersoeke in die
filogeografie van die kus mariene spesies, veral verpligte sittende spesies soos
eendmossels, word vereis om te bepaal of die patroon waargeneem in T. serrata 'n
seldsame geskiedenis het, of nie.
Buur-aansluiting, maksimum parsimonie en Bayesian afleiding op die CO1 geen
het bewyse verskaf vir die monofiletiese afkoms van die genus Tetraclita, maar
Tetraclita spesies wat in Suid Afrika gevind is, deel nie ‘n gemeenskaplike afkoms nie,
wat weer bewyse verskaf vir verskeie kolonisasie gebeure. Hierdie studie het gelei tot
die ontdekking van 'n eksotiese spesie, Balanus perforatus, inheems aan Europa, en die
potensiaal van die indringer om ontwykend te raak aan die ooskus van Suid-Afrika waar
dit gevind is word bespreek.
