Phylogeography of 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

Linckia laevigata

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

Thyca crystallina

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

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 Linckia

Source of

Sum of Variance

Percentage variation Indian vs. Pacific

d.f.

Squares Components

Variation

--------------------------------------------------------------------- 1 35.001 0.13108 Va 2.93

Within Indian OR Pacific

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

FST : 0.08949

0 0 FCT : 0.02933

0.01271

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

Amova Data Showing Variation Across Indian Pacific

Source of

Sum of Variance

Percentage variation

d.f.

Squares Components

Variation ----------------------------------------------------------------------------- Indian vs. Pacific

1 2.229

-0.00455 Va

-0.20

Within Indian OR Pacific

11 27.153

0.01170 Vb

0.51

Within populations

Total Fixation Indices 192 204 FSC : 0.00509

FST : 0.00312

FCT : -0.00198

439.189

468.571

P Value 0.28152

0.30205

0.42522

2.28744 Vc ----------------------------------------------------------------------------- 2.29459

99.69

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

Pairwise Fst values show inverse relationship

0.12

0.1

y = -0.0883x + 0.0363

R 2 = 0.0624

0.08

0.06

0.04

0.02

0 0 0.05

0.1

0.15

Thyca

0.2

0.25

0.3

CONCLUSION What 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 .