A defect in purine nucleotide metabolism in the silkworm, Bombyx mori, causes a translucent larval integument and male infertility

Highlights

• p-oily (op) is a mutation that causes translucent integument and male infertility in the silkworm, Bombyx mori.

• RNA-seq analysis of op mutants identified a candidate gene with an insertional mutation.

• CRISPR/Cas9 targeting of the candidate gene led to the translucent larvae.

• Ingestion of inosine rescued the translucent phenotype of op mutants.

• The op locus governs the conversion of inosine monophosphate to inosine.

Abstract

p-oily (op) is a novel mutant of Bombyx mori exhibiting translucent larval integument and male infertility. Elucidation of the causative gene of the op mutant will help understand the genetic mechanism underlying larval integument coloration and male fertility. Using polymorphisms between B. mori and B. mandarina, the op locus was narrowed down to a 375-kb region. Using RNA-seq analysis, we found that op mutants have a frameshift mutation in the KWMTBOMO13770 gene located in the 375-kb region. A database search indicated that this gene is the human cytosolic 5ʹ-nucleotidase II gene (cN-II) homolog in Bombyx, which mediates the conversion of inosine monophosphate (IMP) to inosine, a precursor of uric acid. CRISPR/Cas9-mediated knockout mutants of the Bm-cN-II gene showed translucent integuments, and there appeared translucent larvae in the crosses between knockout moths and +/op moths. Moreover, the translucent phenotype of, and decreased uric acid content in the larval integument caused by the mutations in the Bm-cN-II gene were rescued by oral administration of inosine. These results indicated that the Bm-cN-II gene is responsible for the op phenotype and that the molecular function of the Bm-cN-II gene is the conversion of IMP to inosine. We also discuss the genetic relationship between the Bm-cN-II gene and male fertility.

Introduction

Animals have a system for the detoxication of toxic ammonia generated by amino acid catabolism. Generally, ammonia is fixed into the amide of glutamine, which works as a primary intermediate in ammonia detoxication. The silkworm, Bombyx mori, possesses two main routes to excrete ammonia. In the first route, glutamine is transported to the silk gland, which has high glutamate synthase activity, which is converted to glutamate (Hirayama et al., 1997; Hirayama and Nakamura, 2002). Glutamate is used as a source of amino acids to synthesize silk protein. In this route, excess ammonia is excreted in the form of silk thread during the spinning stage. In the second route, the excretion of excess nitrogen is linked to the purine nucleotide synthesis pathway. In this pathway, glutamine is a source of nitrogen to form inosine monophosphate (IMP) (KEGG, https://www.genome.jp/kegg-bin/show_pathway?bmor00230) (Fig. 1). IMP is a precursor of uric acid, which is an end product of nitrogen metabolism in terrestrial insects (O'Donnell, 2008). Purine nucleotides are also synthesized from IMP (Fig. 1), suggesting that there is a system for balancing the synthesis of purine nucleotides with the excretion of excess nitrogen.

Although Lepidopteran insects excrete uric acid as feces, uric acid is stored in the larval epidermis as a white pigment and a barrier from sunlight (Lhonoré et al., 1980; Buckner and Newman, 1990; Tamura and Akai, 1999; Timmerman and Berenbaum, 1999; Matsuo and Ishikawa, 1999; Hu et al., 2013). In B. mori, a lack of uric acid in the larval integument results in a translucent integument (Tamura and Akai, 1999). To date, more than 30 loci have been identified that govern the translucent phenotype (SilkwormBase, https://shigen.nig.ac.jp/silkwormbase/top.jsp). The translucency of the larval integuments varies according to specific mutants. Several translucent mutants with a highly translucent integument exhibit male infertility (Tamura et al., 1999). The uncharacterized translucent mutant identified in 1981 and named p-oily (op) is unique because op male moths exhibit fertility defects, although the translucency of the their integument is not very high. Elucidating the causative gene of the op mutation will help understand the genetic mechanism underlying the nitrogen metabolism that governs larval integument coloration and male fertility.

Here, we report then identification of the gene responsible for the op mutant. Using polymorphisms between B. mori and B. mandarina, the op locus was narrowed down to a 375-kb region. Using RNA-seq analysis of the op mutants, we found that the KWMTBOMO13770 gene located in the 375-kb candidate region carried an insertional mutation. A database search indicated that this gene is the human cytosolic 5ʹ-nucleotidase II gene (cN-II) homolog in Bombyx mori, which mediates the 5ʹ dephosphorylation of inosine monophosphate (IMP) to produce inosine, a precursor of uric acid. Knockout mutants of the Bm-cN-II gene that were generated using the CRISPR/Cas9 system revealed that (i) knockout larvae showed moderately translucent integuments and (ii) moderately translucent larvae resulted from the crosses between knockout and +/op moths. Moreover, the translucent phenotype of, and decreased uric acid content in the larval integument caused by the mutations in the Bm-cN-II gene were rescued by oral administration of inosine. These results indicated that the Bm-cN-II gene is responsible for the op phenotype and that the molecular function of the Bm-cN-II gene is the conversion of IMP to inosine. Finally, the genetic relationship between the Bm-cN-II gene and male moth fertility is discussed.