Possible biological processes controlled by the circatidal clock in the mangrove cricket inferred from transcriptome analysis

Intertidal dwellers exhibit a circatidal rhythm corresponding to the tidal cycles, but its molecular mechanism remains largely unknown. The mangrove cricket Apteronemobius asahinai simultaneously exhibits circatidal and circadian rhythms in its locomotor activity. In A. asahinai, it is considered that the circatidal rhythm is generated by the circatidal clock, and its molecular mechanism appears to differ from that of the circadian clock. For the molecular dissection, it is necessary to investigate the effects of a candidate gene disruption in several circatidal rhythms. However, the circatidal rhythms other than locomotor activity have not been examined in this species. Here, to identify biological processes related to the circatidal rhythm, we performed whole-transcriptome analysis using RNA sequencing with RNA samples extracted from whole crickets. Under a light-dark cycle without tidal stimuli, more contigs showed a diel expression pattern than a tidal expression pattern. Gene ontology (GO) enrichment analysis revealed that most GO terms overrepresented in contigs related to the tidal cycle were also overrepresented in contigs related to the diel cycle. GO terms specific to tide-related contigs were related to cuticle development, melanization, and sclerotization. These results suggest that physiological events in the cuticle may be controlled by the circatidal clock.潮间带生物展现出与潮汐周期相匹配的潮汐节律（circatidal rhythm），但其分子机制在很大程度上仍未明确。红树林蟋蟀（Apteronemobius asahinai）的运动活性同时兼具潮汐节律与昼夜节律（circadian rhythm）。在该物种中，潮汐节律被认为由潮汐时钟（circatidal clock）所产生，其分子机制似乎与昼夜节律的分子机制存在差异。为开展分子层面的解析，有必要对若干潮汐节律相关候选基因的敲除效应进行研究，但目前该物种中除运动活性外的其他潮汐节律尚未得到考察。为鉴定与潮汐节律相关的生物学过程，本研究利用从整只蟋蟀中提取的RNA样品开展RNA测序（RNA sequencing），并进行了全转录组分析（whole-transcriptome analysis）。在无潮汐刺激的光暗循环条件下，呈现昼夜表达模式的重叠群（contigs）数量多于呈现潮汐表达模式的重叠群。基因本体（Gene Ontology, GO）富集分析显示，在与潮汐周期相关的重叠群中显著富集的多数基因本体术语，在与昼夜周期相关的重叠群中同样显著富集。与潮汐相关重叠群所特有的基因本体术语涉及表皮发育、黑化作用（melanization）以及硬化作用（sclerotization）。上述结果表明，表皮的生理活动或可由潮汐时钟调控。