Population genetics of Liothyrella neozelanica in Breaksea Sound Erik Suring University of Otago

Population genetics of Liothyrella neozelanica in Breaksea Sound

• Erik Suring

• University of Otago,

• Dunedin, New Zealand

Marine Gene Flow

• Gene flow occurs by larval dispersal

• Larval life history characteristics

• Larval type – passive or swimming
• Duration of planktonic phase
• Settlement characteristics
• Spawning time

• Geographic factors

• Past or present physical barriers to genetic exchange
• Fiord hydrography may limit gene flow

Fiord Dynamics

Liothyrella neozelanica

• Brachiopoda: Articulata: Terebratulidae

• Reported in Fiordland as far north as Doubtful Sound
• Found attached on overhangs
• Aggregated distribution (Richardson 1981)
• Lecithotrophic, brooded larvae (Chuang 1994)
• Found at depths below 20 meters

Terebratella sanguinea

• Articulate brachiopod

• The most common and widely ranged brachiopod in Fiordland
• Less gregarious (Richardson 1981)
• Attached or free-lying
• Found between 5 and 35 meters

Sample Locations

• Inner and Outer locations from Breaksea Sound

• 32 animals from inner location
• 27 animals from outer location
• Collected during summer, 1999

Breaksea Sound

Genetic Differentiation

• No genetic exchange – populations equilibrate at different allele frequencies

• Longer and more complete isolation causes increased fixation of differences

• Genetic markers quantify the amount of genetic variation

• Different markers record different processes and show different scales of variation

Population Questions

• How much genetic variation is there in L. neozelanica?

• Is the population structure of an inner fiord site different from an outer fiord site in Breaksea Sound?

• Is the genetic differentiation (Fst) between inner and outer populations different between species?

• How differently do separate genetic markers resolve genetic population structure?

Allozyme Electrophoresis

• Indirect measure of sequence variation

• Based on electrophoretic changes in proteins that represent allelic changes - phenotypes
• Some loci can be affected by selection, does not register all variation

• Technique is well documented

• Relatively cheap and easy, though polymorphic enzyme systems can be hard to find (Parker et al. 1998)

Allozyme Electrophoresis Methods

• Cellulose-acetate gels

• Homogenize tissues
• Add to gel
• Run gel
• Stain gel

• Hexokinase (Hk)

• Glucose-Phosphate Isomerase (Pgi)

• Peptidase (Pep)

Amplified Fragment Length Polymorphism (AFLP)

• Also an indirect measure of sequence variation

• Cut entire genome with restriction enzymes
• Amplify cut fragments
• Compare size and number of fragments

• AFLP resolves finer differences than allozyme electrophoresis

Allozymes – Hardy-Weinburg Equilibrium

Allozymes – Peptidase Allele Frequencies

Allozymes – Fst

Fst Comparisons

AFLP

• Bands were visible for some animals but overall the gel was underdeveloped

• This could be remedied by

• Using increased radioactivity
• Using a different DNA extraction method

• We have some of the problems have been worked out and may have useful data soon…

Heterozygosity

• Two loci, especially peptidase, were not in Hardy-Weinburg equilibrium

• Banding patterns may not represent allelic variation
• Banding may have been improperly scored
• Natural selection may favor certain alleles
• Inbreeding
• Small populations
• Small sample size

• No conclusions can be made about disequilibrium without further tests

Genetic Differentiation

• There is significant population structure within Breaksea Sound in L. neozelanica.

• Not much allozyme allelic variation (except in peptidase)

• There are also indications that L. neozelanica may have greater within fiord population differentiation than T. sanguinea

• Perhaps because of short planktonic phase and aggregated distribution limits gene flow

Technique Comparison

• Allozymes may evolve too slowly to be informative in this situation

• Fiords have been inundated for less than 10,000 years (Pickrill et al. 1992)

• AFLP may show finer genetic differentiation

• In Lingula anatina DNA sequences have revealed genetic structure not seen with allozymes (Endo et al. 2001)

Further Work

• Find a technique with greater resolution

• L. neozelanica seems to have low variability

• Analyze a greater number of sites

• More inner and outer fiord replicates
• More sites within the fiord

• Make comparisons with different species

• Species with planktotrophic larvae
• Underway by Cecile Perrin and Gigi Ostrow

Conclusions

• There is significant population structure within Breaksea Fiord

• L. neozelanica has low allozyme variability

• L. neozelanica may have greater within fiord differentiation than T. sanguinea

• Further study of the interactions between the fiord environment and larval life histories is promising

Thank you!

• Gigi Ostrow

• Dr. Stephen Wing

• Dr. Michael Roy

• Everyone else in

• Marine Science