genes G C A T T A C G G C A T Review Genetic Resources and Vulnerabilities of Major Cucurbit Crops Rebecca Grumet 1, *, James D. McCreight 2 , Cecilia McGregor 3 , Yiqun Weng 4 , Michael Mazourek 5 , Kathleen Reitsma 6 , Joanne Labate 7 , Angela Davis 8 and Zhangjun Fei 9   Citation: Grumet, R.; McCreight, J.D.; McGregor, C.; Weng, Y.; Mazourek, M.; Reitsma, K.; Labate, J.; Davis, A.; Fei, Z. Genetic Resources and Vulnerabilities of Major Cucurbit Crops. Genes 2021, 12, 1222. https://doi.org/10.3390/ genes12081222 Academic Editor: Alice Hayward Received: 22 June 2021 Accepted: 5 August 2021 Published: 7 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Horticulture, Michigan State University, East Lansing, MI 48824, USA 2 USDA, ARS, Crop Improvement and Protection Research Unit, Salinas, CA 93905, USA; lettucemelon@gmail.com 3 Department of Horticulture and Institute of Plant Breeding, Genetics & Genomics, University of Georgia, Athens, GA 30602, USA; cmcgre1@uga.edu 4 USDA-ARS Vegetable Crops Research Unit, Madison, WI 53706, USA; yiqun.weng@wisc.edu 5 School of Integrative Plant Science, Plant Breeding & Genetics Section, Cornell University, Ithaca, NY 14853, USA; mm284@cornell.edu 6 North Central Regional Plant Introduction Station, Iowa State University, Ames, IA 50014, USA; kathleen.reitsma@usda.gov 7 Plant Genetic Resources Unit, United States Department of Agriculture, Agricultural Research Service, Geneva, NY 14456, USA; joanne.labate@ars.usda.gov 8 Sakata Seed America, Inc., Woodland, CA 95776, USA; angela.davis.phd1@gmail.com 9 Boyce Thompson Institute, Cornell University, Ithaca, NY 14853, USA; zf25@cornell.edu * Correspondence: grumet@msu.edu Abstract: The Cucurbitaceae family provides numerous important crops including watermelons (Citrullus lanatus), melons (Cucumis melo), cucumbers (Cucumis sativus), and pumpkins and squashes (Cucurbita spp.). Centers of domestication in Africa, Asia, and the Americas were followed by distribution throughout the world and the evolution of secondary centers of diversity. Each of these crops is challenged by multiple fungal, oomycete, bacterial, and viral diseases and insects that vector disease and cause feeding damage. Cultivated varieties are constrained by market demands, the necessity for climatic adaptations, domestication bottlenecks, and in most cases, limited capacity for interspecific hybridization, creating narrow genetic bases for crop improvement. This analysis of crop vulnerabilities examines the four major cucurbit crops, their uses, challenges, and genetic resources. ex situ germplasm banks, the primary strategy to preserve genetic diversity, have been extensively utilized by cucurbit breeders, especially for resistances to biotic and abiotic stresses. Recent genomic efforts have documented genetic diversity, population structure, and genetic relationships among accessions within collections. Collection size and accessibility are impacted by historical collections, current ability to collect, and ability to store and maintain collections. The biology of cucurbits, with insect-pollinated, outcrossing plants, and large, spreading vines, pose additional challenges for regeneration and maintenance. Our ability to address ongoing and future cucurbit crop vulnerabilities will require a combination of investment, agricultural, and conservation policies, and technological advances to facilitate collection, preservation, and access to critical Cucurbitaceae diversity. Keywords: Cucurbitaceae; Cucumis; Citrullus; Cucurbita; germplasm; ex situ conservation; genebanks 1. Introduction A limited number of plant families supply the majority of crops that form the basis for human diets. Among these is the Cucurbitaceae family, providing an array of nutritional, flavorful, and colorful crops including watermelons (Citrullus lanatus (Thunb.) Matsum. and Nakai), melons (Cucumis melo L.), cucumbers (Cucumis sativus L.), and pumpkins and squashes (Cucurbita spp.). These crops, which are primarily consumed for fruits and seeds, are noted for remarkable diversity in fruit size, shape, and color, providing both culinary and aesthetic value (Figure 1). Other less widely cultivated cucurbits such as bitter gourd Genes 2021, 12, 1222. https://doi.org/10.3390/genes12081222 https://www.mdpi.com/journal/genes