# Colonization by Distinct Lineages, the Sundaland Barrier, and Historical Bottlenecks Shape the East–West Population Structure of Avicennia Mangroves Across the Indo-Pacific Interface

## TL;DR

This study shows how historical colonization events and geographical barriers shaped the genetic structure of mangrove species in the Indo-Pacific region.

## Contribution

The study identifies Thailand as a contact zone between Indian and West Pacific lineages of Avicennia mangroves and recommends separate conservation management.

## Key findings

- RADseq analysis revealed pronounced East–West genetic divergence between Andaman and Gulf of Thailand populations.
- Low genetic diversity (Ho = 0.073–0.083) and high inbreeding (FIS = 0.169–0.501) were observed due to historical bottlenecks.
- Colonization by distinct lineages and the Sundaland barrier shaped the population structure of Avicennia species.

## Abstract

This genetic study of Avicennia marina, the most widely distributed mangrove species; Avicennia alba; and Avicennia officinalis across the Andaman Sea and the Gulf of Thailand suggests that colonization events by distinct ancestral lineages (Indian and West Pacific Ocean lineages), the Indo-Pacific Barrier (Sundaland), and Pleistocene sea-level fluctuations shaped the East–West population structure and contributed to low genetic diversity and high inbreeding. By highlighting Thailand as a contact zone between Indian and West Pacific Ocean lineages, this study recommends that Andaman and Thai Gulf populations be managed as separate evolutionarily significant units for conservation management.

The emergence of Sundaland during the Pleistocene glaciation has played a crucial role, as the Indo-Pacific Barrier (IPB), in shaping the genetic structure of marine taxa and coastal flora, specifically mangroves. This study investigated the genetic diversity, population structure, demographic history and phylogeography of Avicennia marina and two other Indo-West Pacific (IWP) Avicennia species, Avicennia alba and Avicennia officinalis, across the Andaman Sea (Indian Ocean) and the Gulf of Thailand (Pacific Ocean). Using Restriction-site-Associated DNA sequencing (RADseq), we generated thousands of genome-wide SNPs for 362 Avicennia individuals and revealed a pronounced East–West genetic divergence, separating the Andaman and Gulf of Thailand populations. Phylogeographic and demographic analyses suggest that colonization events by distinct ancestral lineages (Indian and West Pacific Ocean lineages), the Indo-Pacific Barrier (Sundaland), and Pleistocene sea-level fluctuations shaped the population structure and contributed to low genetic diversity (Ho = 0.073–0.083) and high inbreeding coefficients (FIS = 0.169–0.501). This study highlights the importance of Thailand, as part of the Indo-Pacific interface, in harboring genetic resources from both Indian and West Pacific Ocean lineages, as exemplified in A. marina. Consequently, Andaman and Thai Gulf populations should be managed as distinct evolutionarily significant units (ESUs).