*Article* **Comparative Analysis of Calcium-Dependent Protein Kinase in Cucurbitaceae and Expression Studies in Watermelon**

#### **Chunhua Wei** † **, Ruimin Zhang** † **, Xiaozhen Yang, Chunyu Zhu, Hao Li, Yong Zhang, Jianxiang Ma, Jianqiang Yang and Xian Zhang \***

State Key Laboratory of Crop Stress Biology in Arid Areas, College of Horticulture, Northwest A&F University, Yangling 712100, China; xjwend020405@nwafu.edu.cn (C.W.); zrm0923@163.com (R.Z.); yxzh5186@126.com (X.Y.); zhucy@nwafu.edu.cn (C.Z.); yuanyilihao123@163.com (H.L.);

zhangyong123@nwafu.edu.cn (Y.Z.); majianxiang@126.com (J.M.); yangjq1208@126.com (J.Y.)

**\*** Correspondence: zhangxian@nwsuaf.edu.cn; Tel.: +86-029-8708-2613

† These authors contributed equally to this work.

Received: 21 March 2019; Accepted: 20 May 2019; Published: 23 May 2019

**Abstract:** Both the calcium-dependent protein kinases (CDPKs) and CDPK-related kinases (CRKs) play numerous roles in plant growth, development, and stress response. Despite genome-wide identification of both families in *Cucumis*, comparative evolutionary and functional analysis of both *CDPKs* and *CRKs* in Cucurbitaceae remain unclear. In this study, we identified 128 *CDPK* and 56 *CRK* genes in total in six Cucurbitaceae species (*C. lanatus*, *C. sativus*, *C. moschata*, *C. maxima*, *C. pepo*, and *L. siceraria*). Dot plot analysis indicated that self-duplication of conserved domains contributed to the structural variations of two *CDPKs* (*CpCDPK19* and *CpCDPK27*) in *C. pepo*. Using watermelon genome as reference, an integrated map containing 25 loci (16 *CDPK* and nine *CRK* loci) was obtained, 16 of which (12 *CDPK* and four *CRK*) were shared by all seven Cucurbitaceae species. Combined with exon-intron organizations, topological analyses indicated an ancient origination of groups CDPK IV and CRK. Moreover, the evolutionary scenario of seven modern Cucurbitaceae species could also be reflected on the phylogenetic trees. Expression patterns of *ClCDPKs* and *ClCRKs* were studied under different abiotic stresses. Some valuable genes were uncovered for future gene function exploration. For instance, both *ClCDPK6* and its ortholog *CsCDPK14* in cucumber could be induced by salinity, while *ClCDPK6* and *ClCDPK16*, as well as their orthologs in *Cucumis*, maintained high expression levels in male flowers. Collectively, these results provide insights into the evolutionary history of two gene families in Cucurbitaceae, and indicate a subset of candidate genes for functional characterizations in the future.

**Keywords:** calcium-dependent protein kinases; CDPK-related kinases; evolutionary analysis; expression pattern; abiotic stress; cucurbitaceae
