# Phylogenetic History and Adaptation Mechanisms of Casuarina equisetifolia Landraces in China

## TL;DR

This study explores how Casuarina equisetifolia trees in China adapted quickly to new environments through genetic analysis and hybridization with another species.

## Contribution

The study reveals new insights into the genetic structure and adaptive mechanisms of Chinese Casuarina equisetifolia landraces through hybridization and genomic variation.

## Key findings

- Chinese Casuarina equisetifolia landraces show genetic structures similar to Southeast Asian populations.
- 76.7% of individuals exhibit admixture with Casuarina glauca, indicating frequent hybridization.
- Integrated genomic signals in hybrids suggest adaptive traits from both parent species.

## Abstract

Following its introduction to China,
Casuarina equisetifolia
rapidly adapted to its new habitat and was extensively cultivated as a crucial tree species for coastal shelterbelts. However, the systematic classification of Chinese landraces remains controversial and the mechanism underlying the rapid adaptation of this species in China remains unknown. To understand more fully the phylogenetic history and adaptation mechanisms of
C. equisetifolia
, 66 samples were collected from Chinese landraces and five from other potentially related provenances. Single nucleotide polymorphisms were detected through genotyping by sequencing and used for genetic structural analysis and genome comparison. The genetic structures of Chinese landraces were largely consistent with that of
C. equisetifolia
in natural and other distributions, while 76.7% of individuals showed clear admixture with
Casuarina glauca
, indicating frequent spontaneous interspecific introgression or hybridization. Phylogenetic and gene flow analyses further support a distinct local origin involving unique parental combinations. Genomic variation associated with pathogen–plant interactions was observed between natural provenances and founder populations along migration pathways. Moreover, integrated genomic signals were identified in hybrids, including secondary metabolite synthesis derived from
C. glauca
, as well as growth regulation and metabolic processes inherited from
C. equisetifolia
, offering preliminary insights into adaptive mechanisms during the introduction process. These findings enhance our understanding of the phylogenetic history of Casuarina landraces in China and may contribute to improved efficiency in domestication and breeding programs.

Genetic analysis revealed that Chinese
Casuarina equisetifolia
landraces exhibit structures similar to Southeast Asian populations but also show evidence of hybridization with
C. glauca
. The study detected genomic variations linked to pathogen interactions along migration pathways and identified integrated adaptive signals in hybrids, including traits from both parent species related to secondary metabolites and growth regulation. These findings clarify the phylogenetic history of Chinese casuarina landraces and provide insights into their rapid adaptation, potentially aiding future domestication and breeding efforts.