## 4 Discussion  
In the present study, 59,902 _de novo_ assembled unigenes were obtained. Moreover, the results of BUSCO showed 94.39% of the genome was complete. These results showed the quality of transcriptome in our study was high, and it was reliable for subsequent analyses. Among the unigenes, 63.45% were annotated to public databases. The match ratio was low because of the lack of genetic data of lizardfish species in the public databases. To date, only three protein sequences have been deposited for species in genus _Saurida_ in the NCBI NR database, explaining the low number of BLAST top hits for lizardfish. The result of main species distribution matched against the NR database showed that 11.99% of the annotated unigenes shared similar sequences with _Lates calcarifer_, whose draft genome was published in 2015 [37]. _Seriola lalandi dorsalis, Larimichthys crocea, Seriola dumerili_ and _Stegastes partitus_ also shared similar sequences with _S. elongata_. Thus, unigenes of _S. elongata_ transcriptome matched well to proteins of other teleost fish. It has been reported that the annotation results of the non-model species were highly dependent on the availability of annotated sequence information in the database, and the sizes of their contig sequences [38; 39]. Compared to other non-model teleost species, the number of transcripts obtained for _S. elongata_ from this study was at moderate level [24; 26; 40]. It is evident that genes involved in gonad development and related to sex differentiation play important roles in controlling the sex ratio of teleost fishes [24]. It is crucial to elucidate the mechanisms of sex determination and gonad differentiation. In the present study, we identified numerous DEGs and suspected that these genes may play important roles in the sex differentiation between genders of _S. elongata_. However, sex determination in teleost fish is an extremely complex process, regulated by numerous genes [9; 25]. Due to the limitations of the present study, we could not clarify the exact mechanisms of sex determination or gonad differentiation. The transcriptome data in this study will facilitate  
Figure 7: Expression levels of eight unigenes as determined by transcriptome analysis and qRT-PCR. further studies of _S. elongata._ With the annotation results, we identified several well-known genes relate to sex control and gonadal development (_zp_, _dmtr_, _42 sp43_, _start_, _Weel_, _foxl2_, etc.). The zona pellucida plays many important biological functions, including postfertilization blockade of polyspermy and relative species-specific binding of sperm to ovulated eggs [41; 42]. In fish oocytes, the _zona pellucida_ plays a protective role in sperm binding [43]. It has been reported that zona pellucida protein 3 plays a crucial role in acrosome reaction, and zona pellucida 4 participates in primordial follicle formation [44; 45]. In the present study, _zp 1-4_ showed significantly higher expression in females, showing these genes may play important roles in reproduction and folliculogenesis in _S. elongata._  
In teleost, _dmtr_ gene families play crucial roles in sexual differentiation and determination [46; 47]. It is well known that the _dmtr_ gene family includes many members. These genes encode putative transcription factors with evolutionarily well-conserved Doublesex and Mad-3 (DM) domains [48]. The DM domain is involved in sexual differentiation and development in many species (insects, fish and mammals) [49; 50]. Eight _dmtr_ genes (_dmtr1_ through _dmtr_ 8) have been reported in mammals [51], and some of them have also been detected in teleosts. These include _Oroochromis niloticus_[52], _Gadus morhua_[53], _Danio rerio_[54], _Takifugu rubripes_[55] and _Cynoglosss semilaevis_[56]. As an important member, the _dmtr1_ gene plays a key role in neurogenesis, testicular hypoplasia and even embryonic sex reversal [57; 58]. Recently, the _dmtr2_ gene was found to play a crucial factor in germ cell maturation and gonadal differentiation in male teleost fishes [56].