Primary process of speciation in marine lakes of Palau Islands:The case of the striped silverside, Atherinomorus endrachtensis

(Atherinidae, Atheriniformes, Teleostei)

Ryo Gotoh, N. Hanzawa Graduate School of Science and Engineering, Yamagata University

Yamagata University

Terrestrial organisms

Marine organisms

Background

Primary process of speciation of marine organisms caused by geographical isolation is unclear

Ex) Darwin’s finch on Garapagos Islands

・ Ocean is connected in the world・ Gene flow is occurring at the large scale

Many study of geographical isolation have conducted.It is easy to set up geographical isolation models, such as oceanic islands.

Ex) Speciation caused by formation of the Isthmus of Panama

・ This vicariant event had occurred several million years ago

We focused on marine lakes on Palau Islands

・ Palau Islands are located in the West Pacific

・ There are approximately 300 islands

・ The limestone islands called “Rock Islands” are in the central and south parts of Palau

・ There are approximately 70 marine lakes in Palau Islands

Palau Islands and marine lakes

・ Several marine lakes have anoxic layer (meromictic lakes)

・ Toxic substances (ex. H2S) accumulate in anoxic layer

・ Organisms could not migrate between marine lake and lagoon in the present

The formation procedure of marine lakes

Comparing genetic structures of lagoon populations and marine lake populations, we show primary process of speciation of

marine organisms caused by geographical isolation.

“Geographical isolation model for marine organisms”

Organisms inhabiting marine lakes have been isolated since the lakes were formed

Biology Found in shallow coastal waters

Atherinomorus endrachtensis

(Atherinidae, Atheriniformes, Teleostei)

Distribution Western Central Pacific: Philippines and Micronesia to northern Australia and the Solomon Islands.

Materials & Methods

We conducted population genetic analyses based on mt DNA cyt b gene (1141bp) andcontrol region (877bp)

Ongael Island

JFL

MEC3

ONG

Mecherchar Island

Red letter shows marine lakes

Bule letter shows lagoon

１０ｋｍ

(20)

(24)

(26) KBB(23)

JFOS

NTOS(14)

HLOS(15)

(20)

Palau Islands

Sulawesi

Results & DiscussionResults & Discussion

JFOS 20 　　　　 6 　　 0.737 0.089

KBB 23 　　　　 9 0.688 0.140

NTOS 　　 14 　　　　 6 0.747 0.099

HLOS 　　 15 　　　 4 0.371 0.035

　　 33 　　　 17 0.936 0.335

23 　　　　 21 0.988 0.642

13 　　　　 13 1.000 0.680

16 　　　 12 0.967 0.519

Genetic diversity

JFL 　　 20 　　　　 4 0.284 0.026

MEC3 24 　　　　 1 0.000 　 0.000

ONG 　　 26 　　　　 2 0.148 0.013

　　 25 　　　　 8 　　 0.590 0.120

24 　　　　 4 0.308 0.047

26 　　　　 3 0.151 0.026

N H h π

Cyt b

N H h π

Control region

N: number of individuals, H: number of haplotypes, h: haplotype diversityπ: nucletide diversity

0.5

0.5

0.5

0.5

h

π

Cyt b Control region

Genetic diversity of marine lake populations is much lower than that of lagoon populations

lagoon

Marine lakeπ

h

JFLMEC3

KBBJFOSNTOSHLOS

ONG

Marine lake

Lagoon

Statistical parsimony network based on cyt b gene

Palau

Gene flow occurs at large scale

Palau-Sulawesi =1,200km

The relationships among marine lake populations are unclear.

Sulawesi

ONGJFL

MEC3

Statistical parsimony network based on control region

JFLMEC3 KBB

JFOSNTOSHLOS

ONG

Marine lake Lagoon

Sulawesi

・ One major haplotype was found

・ The major haplotype was also found in lagoon population

２４

１１

Founder effect

The ONG population

Isolation Genetic drift

Founder effect

Fixation

The ONG population

・ One major haplotype was found

・ The major haplotype was also found in lagoon population

・ Two endemic haplotypes were found

２４

１１

The JFL and MEC3 populations

CT repeat５⇔４

・ The number of times of CT repeat found in the JFL and the MEC3 population differs from lagoon population

・ The minor haplotypes was endemic in each marine lake and the haplotypes were derived from the major haplotype

・ One major haplotype and several minor haplotypes were found

２０１６

３

２

This strongly suggests that the minor haplotypes were emerged in marine lakes

JFL

MEC3

Lagoon population has a large number of haplotypes

It is unlikely that same haplotype was fixed in different marine lake populations by chance.However, the JFL and MEC3 population have the same major haplotype.

We consider “stepwise formation hypothesis”, focusing on the facts that the two marine lakes are in the same island and differ in depth (JFL: 30m, MEC3: 20m)

Migration from lagoon to JFL

Haplotype was fixed by founder effect

Each marine lake population is isolated by anoxic layer

Migration from JFL to MEC3

Fixation index st

0.059 (P<0.05)

Slightly genetic differentiation has already occurred between the JFL and the MEC3 population

Stepwise formation hypothesis

lagoon population

Ancestral

Colonized?JFL population

MEC3 population

ONG population

Founder effct

　　

New mutationAccumulated

~12,000 years ago present

Population growth

Founder effct

Population growth

Population growth

A schematic showing the primary process of speciation in marine lake populations

Conclusion

Marine lakes are excellent model for geographical isolation of marine organisms

・ Genetic structure between lagoon and marine lake populations are obviously different

・ Genetic structure among marine lake populations are also different

・ Each marine lake population has endemic haplotypes

Thank you very for your attention!!