Stacking microscopy images of the pteropod Limacina bulimoides

Pteropods, a group of holoplanktonic gastropods, are regarded as
bioindicators of the effects of ocean acidification on open ocean
ecosystems, because their thin aragonitic shells are susceptible to
dissolution. While there have been recent efforts to address their
capacity for physiological acclimation, it is also important to gain
predictive understanding of their ability to adapt to future ocean
conditions. However, little is known about the levels of genetic variation
and large scale population structuring of pteropods, key characteristics
enabling local adaptation. We examined the spatial distribution of genetic
diversity in the mitochondrial cytochrome c oxidase I (COI) and nuclear
28S gene fragments, as well as shell shape variation, across a latitudinal
transect in the Atlantic Ocean (35°N-36°S) for the pteropod Limacina
bulimoides. We observed high levels of genetic variability (COI π = 0.034,
28S π = 0.0021) and strong spatial structuring (COI ΦST = 0.230, 28S ΦST =
0.255) across this transect. Based on the congruence of mitochondrial and
nuclear differentiation, as well as differences in shell shape, we
identified a primary dispersal barrier in the southern Atlantic
subtropical gyre (15-18°S). This barrier is maintained despite the
presence of expatriates, a gyral current system, and in the absence of any
distinct oceanographic gradients in this region, suggesting that
reproductive isolation between these populations must be strong. A
secondary dispersal barrier supported only by 28S pairwise ΦST comparisons
was identified in the equatorial upwelling region (between 15°N-4°S),
which is concordant with barriers observed in other zooplankton species.
Both oceanic dispersal barriers were congruent with regions of low
abundance reported for a similar basin-scale transect that was sampled two
years later. Our finding supports the hypothesis that low abundance
indicates areas of suboptimal habitat that result in barriers to gene flow
in widely-distributed zooplankton species. Such species may in fact
consist of several populations or (sub)species that are adapted to local
environmental conditions, limiting their potential for adaptive responses
to ocean changes. Future analyses of genome-wide diversity in pteropods
could provide further insight into the strength, formation and maintenance
of oceanic dispersal barriers.

翼足类（Pteropods）是一类终生浮游腹足类，因其薄而脆弱的文石质贝壳易发生溶解，被视为海洋酸化对开阔大洋生态系统影响的生物指示物。尽管近期已有诸多研究探讨其生理驯化能力，但解析其适应未来海洋环境的预测性机制同样至关重要。然而，目前对于翼足类的遗传变异水平与大尺度种群结构——这是实现本地适应的关键特征——的认知仍十分有限。本研究以大西洋（35°N-36°S）纬度样带中的尖笔帽螺（Limacina bulimoides）为研究对象，分析了其线粒体细胞色素c氧化酶I（COI）与核28S基因片段的遗传多样性空间分布特征，以及贝壳形态变异情况。研究结果显示，该纬度样带内翼足类具有较高的遗传变异度（COI π=0.034，28S π=0.0021），且存在显著的空间种群结构（COI ΦST=0.230，28S ΦST=0.255）。结合线粒体与核基因分化的一致性，以及贝壳形态的差异，本研究在南大西洋副热带环流（15°S-18°S）区域识别出一处主要扩散障碍。尽管该区域存在异地迁入个体与环流系统，且未观测到显著的海洋学梯度，但该扩散障碍依然存在，这提示种群间的生殖隔离程度较强。此外，本研究在赤道上升流区（15°N-4°S）识别出一处仅由28S成对ΦST比较支持的次要扩散障碍，这一结果与其他浮游动物物种中观测到的扩散障碍一致。两处大洋扩散障碍均与两年后开展的同类盆尺度样带调查中报道的低丰度区域相吻合。本研究结果支持以下假说：在广分布浮游动物物种中，低丰度区域代表亚适宜生境，这类生境会形成基因流障碍。此类物种实际上可能包含多个适应局部环境的种群或（亚）物种，这限制了其对海洋环境变化的适应性响应潜力。未来针对翼足类全基因组多样性的分析，可进一步深入解析大洋扩散障碍的强度、形成与维持机制。