Abstract
The Aulonocara baenschi is a species inhabiting Lake Malawi and its population has decreased by over 70% over the past 10 years. Since 2018, the species has been included in the IUCN Red List and its status has been set to the critically endangered (CR) level. And the fish is known to have rare but naturally occurring albino strain. Despite that, little has been done to develop its genetic resources. Accordingly, this study performed high-throughput sequencing on the caudal fin of the two different-colored A. baenschi in order to identify their pigment-related genes and analyze the expression patterns of the genes on the transcriptome level. Through the Illumina sequencing platform, we obtained 77,035,702 and 88,689,756 reads from the caudal fin of the wild-type and the albino-type, respectively. In addition to that, using de-novo assembly and various annotation databases, a total of 141,116 contigs, including 129,369 unigenes were obtained. In the comparative analysis of differentially expressed genes between the wild-type and the albino-type in terms of melanogenesis, the tyrosinase gene family involved in melanin synthesis tended to be less expressed in the albino-type than the wild-type. However, the expression of the critical proteins involved in the pigment synthesis of xanthophore was higher in the albino-type than in the wild-type. In addition, the yellow-orange pigment was more apparently expressed in the caudal fin of the albino-type than of the wild-type. This indicates that the external characteristics of the fish were in accordance with the analysis results on the transcriptome level. The results of this study will be the foundation of research on expression characteristics and gene networking analysis of the pigmentation-related genes of fish.
Highlights
Albinism, characterized by a lack of melanin pigment in the skin, hair, and eye [oculocutaneous albinism (OCA) or ocular albinism (OA)], is known to be associated with mutations in genes involved in melanin synthesis (Oetting and King, 1999)
Additional annotations were conducted using Gene ontology (GO), GO-Slim, and EggNOG, resulting in a total of 31,954 contigs (24.7%). 70.3% of the annotated genes belong to the same family as the fish used in this experiment and are African cichlids species: Astatotilapia calliptera (24.8%), Maylandia zebra (22.2%), Haplochromis burtoni (12.2%), and Pundamilia nyererei (11.1%) (Supplementary Figure S2)
It is interesting to note that the expression of DCT, MITFA, and TYR genes involved in melanin synthesis is lower in the red-colored crucian carp than in the back-gray colored crucian carp, which tends to be correlated with DNA methylation levels (Zhang et al, 2017a)
Summary
Albinism, characterized by a lack of melanin pigment in the skin, hair, and eye [oculocutaneous albinism (OCA) or ocular albinism (OA)], is known to be associated with mutations in genes involved in melanin synthesis (Oetting and King, 1999). Albinism is a phenomenon which can be observed in a human body and in the body of a fish It has long been reported and until recently, studies have been conducted on various species with albinism (Johnson, 1968; Gong et al, 2019). In the case of fish, more varied kinds of pigment cell types have been found than in other species. Due to the diversity of pigment cells, fish have a wide range of body color and make use of them. It has been reported that inbreeding in the same species is one of the causes of body-color diversity (Shikano et al, 2007; Ohshima et al, 2013; Hattori et al, 2020)
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