Comparative genome size estimation of different life stages of grey mullet, Mugil cephalus Linnaeus, 1758 by flow cytometry

Abstract

CThe striped grey mullet, Mugil cephalus, is the most widespread and economically important high-value species among the family Mugilidae (Order: Mugiliformes), which comprises a total of 26 valid genera and 79 valid species (Eschmeyer et al., 2021). They occupy the lower level of food chain and are abundantly distributed throughout the tropic, subtropic and temperate regions extending geographi-cally between the latitudes of 42° North and 42° South in all coastal waters of the world (Whitfield et al., 2012). M. cephalus is a eury-haline species, which can tolerate wide ranges of salinities ranging from freshwater to hypersaline (Cardona, 2006). The species is also known to survive in a wide range of dissolved oxygen levels and is found in varied turbidity levels and benthic substratum (Whitfield et al., 2012). These eurytopic characteristics together with the meat quality and huge demand for mullet roe make it a popular fishery and candidate species for brackishwater aquaculture. In India, the farming of this fish has been reported since ancient times and has been extensively cultured in the brackishwater areas in many parts of the country since 1947 (Fao, 2021), exclusively depending on the natural fry collected from wild. Recently, the induced breeding of M. cephalus has been standardized in India (Sukumaran et al., 2021). M. cephalus is often confused with other congeneric species such as Mugil liza (now Mugil platanus) because of similar morphometric characteristics (Menezes et al., 2010) and is considered as a part of a species complex (Shen et al., 2011) comprising at least 14 Mugil sp. (Durand et al., 2012).Owing to recent advancements made in next-generation se-quencing (NGS), whole-genome sequences of many aquaculture fish and shellfish have been deciphered. The genome data generated are not only used in basic research such as systematics, evolution and comparative genomics, but also applied sciences in aquaculture and fisheries such as genetic resources management and selective breeding (Lu & Luo, 2020). When no reference genome exists for a species, prior estimation of genome size is essential to understand the genome content and the read depth in NGS (Swathi et al., 2018). There are several methods used to estimate the genome size in fish and shellfish species including real-time quantitative PCR (Wilhelm et al., 2003), Feulgen image analysis densitometry (FIAD; Jeffery & Gregory, 2014) and flow cytometry (Zhu et al., 2012), of which flow cytometry is considered as the best method for accurate estimation of genome size (Hare & Johnston, 2012). Previous reports have doc-umented a range of genome size estimated in Mugil cephalus. For example, the estimated M. cephalus genome size was ~1.02 Gb in female fish (Dor et al., 2020), ~1.1 Gb in male fish (Dor et al., 2016), 0.81 pg (0.79 Gb) (Hardie & Hebert, 2004; Ojima, 1990) and 0.99 pg (0.97 Gb) (Hinegardner & Rosen, 1972). Many studies conducted so far tried to correlate genome size of fish with phenotype, ecological