Phylogeographyof the Seastar Linckia laevigata and its obligate parasite Thyca

crystallina

Bolanle Akinronbi, Marta Muñoz, Paul Barber

What is Phylogeography?

Phylogeography deals with the historical, phylogenetic components of the spatial distribution of gene lineages; so that time and space are jointly considered axes onto which gene genealogies of interest are mapped (Avise, 2000).

Linckia laevigata is a species of sea star that lives on a coral reef.

Linckia laevigata

Thyca crystallina is a species of gastropod snails. They are obligate parasites of the species Linckia laevigata.

Thyca crystallina

Why Study Host-Parasite Relationships?Information obtained from phylogeographic studies of Host-Parasite relationships can be used to better understand the ecology of both host and parasite as well as provide information about effective population sizes, dispersal distances, gene flow rates and the factors that control these parameters. ( McCoy et al). Knowledge thus obtained may then be applied on a grander scale in areas such as conservation, vaccine development and disease prevention.These studies are also important for directing future studies of cospeciation and coevolution of Host and parasite species

Previous comparative studies of Host-Parasite populations have revealed varying results. In some cases both Host and parasite show equal phylogeographic structure, in some the host had more structure while parasites were more structured in others. (McCoy et al 2005).

Coevolution Host and Parasites

Hafner et al (1994)

HYPOTHESIS

Linckia disperses as larvae and so does its parasite Thyca, therefore it is expected that they will have similar patterns of phylogeography.

The aim of the study was twofold:

1. to investigate genetic relationships within Linckia and Thyca populations from different locations in the Indo-West Pacific Ocean. 2. to compare patterns of phylogeography between both Host-Parasite species

METHODS 377 individuals of the host Linckia were

sampled in 25 populations from locations in the coral reefs around the Indo-West Pacific and 204 individuals of thyca were sampled from 13 populations.

DNA extraction using chelex PCR Amplification of the mitochondrial gene:

Cytochrome Oxidase 1 (COI) They were sequenced on a ABI 377 DNA

Sequencer

ANALYSIS

Individual haplotype definitions were obtained using MacClade.Genetic analysis was conducted using PAUP and ARLEQUIN.

LINCKIA LINCKIA

THYCA

LINCKIA

THYCA

Amova Data Showing Variation In LinckiaSource of Sum of Variance Percentage variation d.f. Squares Components Variation----------------------------------------------------------------------------------------------------------------------------------------------------

1 35.001 0.13108 Va 2.93

21 181.828 0.26882 Vb 6.02

Within 373 1517.555 4.06851 Vc 91.05 populations---------------------------------------------------------------------- ---------------------------------------------------------------------- Total 395 1734.384 4.46841---------------------------------------------------------------------- ---------------------------------------------------------------------- Fixation Indices Pvalues FSC : 0.06198 0 FST : 0.08949 0 FCT : 0.02933 0.01271---------------------------------------------------------------------- ----------------------------------------------------------------------

Within Indian OR

Pacific

Indian vs. Pacific

Amova Data Showing Variation Across Indian PacificSource of Sum of Variance Percentage variation d.f. Squares Components Variation

------------------------------------------------------------------------------

1 2.229 -0.00455 Va -0.20

11 27.153 0.01170 Vb 0.51

Withinpopulations 192 439.189 2.28744 Vc 99.69------------------------------------------------------------------------------Total 204 468.571 2.29459------------------------------------------------------------------------------ Fixation Indices P Value FSC : 0.00509 0.28152 FST : 0.00312 0.30205 FCT : -0.00198 0.42522

Indian vs. Pacific

Within Indian OR

Pacific

y = -0.0883x + 0.0363R2 = 0.0624

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.05 0.1 0.15 0.2 0.25 0.3Thyca

Linkia

Pairwise Fst values show inverse relationship

CONCLUSIONWhat have we found?

Thyca is not coevolving with Linckia. Rather its pattern of gene flow suggests that it may have evolved ways to disperse beyond the dispersal distances of its host, Linckia.

This may not be surprising given the difference in terrestrial vs marine methods of Host - parasite Dispersal.

It will be interesting to see in future studies how the length of larval stages in both Linckia and Thyca may have influenced the differences observed in their phylogeographic patterns.

ACKNOWLEDGEMENTS

I’d like to thank Paul Barber, Marta Munoz, Elizabeth Jones and Eric Crandall for their support and contributions towards this project.