Carotenoid metabolic profiling and transcriptome-genome mining reveal functional equivalence among blue-pigmented copepods and appendicularia

The tropical oligotrophic oceanic areas are characterized by high water transparency and annual solar radiation. Under these conditions, a large number of phylogenetically diverse mesozooplankton species living in the surface waters (neuston) are found to be blue pigmented. In the present study, we focused on understanding the metabolic and genetic basis of the observed blue phenotype functional equivalence between the blue pigmented organisms from the phylum Arthropoda, subclass copepoda (Acartia fossae) and the phylum Chordata, class appendicularia (Oikopleura dioica) in the Red Sea. Previous studies have shown that carotenoid protein complexes are responsible for blue coloration in crustaceans. Therefore, we performed carotenoid metabolic profiling using both targeted and non-targeted (high-resolution mass spectrometry) approaches in four different blue-pigmented genera of copepods and one blue-pigmented species of appendicularia. Astaxanthin was found to be the principal carotenoid in all species. The pathway analysis showed that all species can synthesize astaxanthin from β-carotene, ingested from dietary sources, via 3-hydroxyechinenone, canthaxanthin, zeaxanthin, adonirubin or adonixanthin. Further, using de novo assembled transcriptome of blue A. fossae (subclass copepoda) we identified highly expressed homologous β-carotene hydroxylase enzymes and carotenoid binding proteins responsible for astaxanthin formation and the blue phenotype. In blue O. dioica (class appendicularia), corresponding putative genes were identified from the reference genome. Collectively, our data provide molecular evidences for the bioconversion and accumulation of blue astaxanthin-protein complexes underpinning the observed ecological functional equivalence and adaptive convergence among neustonic mesozooplankton.

热带寡营养大洋海域以水体高透明度与年太阳辐射总量充沛为典型特征。在此环境条件下，栖息于表层水体的浮游生物（neuston）类群中，存在大量系统发育多样性丰富的中型浮游动物（mesozooplankton）物种，其体色呈蓝色。本研究聚焦于解析红海海域中，节肢动物门（Arthropoda）桡足亚纲（Copepoda）长腹剑水蚤（Acartia fossae）与脊索动物门（Chordata）尾海鞘纲（Appendicularia）异体住囊虫（Oikopleura dioica）这两类蓝色素沉着生物之间，蓝色表型功能等价性的代谢与遗传基础。既往研究表明，类胡萝卜素（carotenoid）-蛋白质复合物是甲壳类动物呈现蓝色体色的核心成因。据此，本研究针对4个不同蓝色素沉着桡足类属群，以及1个蓝色素沉着尾海鞘纲物种，采用靶向与非靶向（高分辨质谱（high-resolution mass spectrometry））联用的方法开展类胡萝卜素代谢谱分析。研究发现，所有受试物种的优势类胡萝卜素均为虾青素（astaxanthin）。代谢通路分析显示，所有受试物种均可通过3-羟基海胆酮、角黄素、玉米黄质、阿多红素（adonirubin）与阿多黄质（adonixanthin），将从膳食中摄取的β-胡萝卜素（β-carotene）转化合成虾青素。进一步地，本研究对蓝色长腹剑水蚤（桡足亚纲）进行从头组装转录组（transcriptome）分析，鉴定出了高表达的同源β-胡萝卜素羟化酶与类胡萝卜素结合蛋白，这类蛋白参与虾青素的合成及蓝色表型的形成。针对蓝色异体住囊虫（尾海鞘纲），本研究从其参考基因组中鉴定出了对应的候选功能基因。综上，本研究的数据为蓝色虾青素-蛋白质复合物的生物转化与积累提供了分子证据，该复合物正是浮游中型浮游动物间所观测到的生态功能等价性与适应性趋同现象的核心分子基础。