Migration trajectories of the diamondback moth Plutella xylostella in China inferred from population genomic variation

BACKGROUND: The diamondback moth (DBM), Plutella xylostella (Lepidoptera:
Plutellidae), is a notorious pest of cruciferous plants. In temperate
areas, annual populations of DBM originate from adult migrants. However,
the source populations and migration trajectories of immigrants remain
unclear. Here, we investigated migration trajectories of DBM in China with
genome-wide single nucleotide polymorphisms (SNPs) genotyped using
double-digest RAD (ddRAD) sequencing. We first analyzed patterns of
spatial and temporal genetic structure among southern source and northern
recipient populations, then inferred migration trajectories into northern
regions using discriminant analysis of principal components (DAPC),
assignment tests and spatial kinship patterns. RESULTS: Temporal genetic
differentiation among populations was low, indicating sources of recipient
populations and migration trajectories are stable. Spatial genetic
structure indicated three genetic clusters in the southern source
populations. Assignment tests linked northern populations to the Sichuan
cluster, and central-eastern populations to the South and Yunnan clusters,
indicating that Sichuan populations are sources of northern immigrants and
South and Yunnan populations are sources of central-eastern populations.
First-order (full-sib) and second-order (half-sib) kin pairs were always
found within populations, but about 35-40% of third-order (cousin) pairs
were found in different populations. Closely related individuals in
different populations were in about 35-40% of cases found at distances of
900 to 1500 km, while some were separated by over 2000 km. CONCLUSION:
This study unravels seasonal migration patterns in the DBM. We demonstrate
how careful sampling and population genomic analyses can be combined to
help understand cryptic migration patterns in insects.

研究背景：小菜蛾（diamondback moth, DBM）学名Plutella xylostella，隶属于鳞翅目菜蛾科，是十字花科植物的重大农业害虫。在温带区域，小菜蛾的年度种群均源自成虫迁飞个体，但外来迁飞种群的源群体与迁飞轨迹仍不明确。本研究利用双酶切限制性位点关联DNA（double-digest RAD, ddRAD）测序获取基因型的全基因组单核苷酸多态性（single nucleotide polymorphisms, SNPs），对中国境内小菜蛾的迁飞轨迹展开探究。我们首先分析了南部源种群与北部受体种群间的时空遗传结构模式，随后通过主成分判别分析（discriminant analysis of principal components, DAPC）、群体分配检验与空间亲缘关系模式，推断了北部区域的迁飞来源。
研究结果：种群间的时间遗传分化程度较低，表明受体种群的来源与迁飞轨迹较为稳定。空间遗传结构分析显示，南部源种群可划分为3个遗传聚类群。群体分配检验结果表明，北部种群与四川聚类群存在关联，中东部种群则与南部及云南聚类群相关联，这说明四川种群是北部迁飞个体的源种群，而南部与云南种群为中东部种群的源群体。一级亲缘关系（全同胞）与二级亲缘关系（半同胞）的个体对始终存在于同一种群内，但约35%-40%的三级亲缘关系（表/堂兄妹）个体对分布于不同种群中。不同种群间的密切亲缘个体中，约35%-40%的空间距离处于900至1500 km之间，部分个体间的距离甚至超过2000 km。
研究结论：本研究阐明了小菜蛾的季节性迁飞模式。我们证实，通过严谨的采样设计与群体基因组学分析相结合，可有效解析昆虫中隐秘的迁飞模式。