Introduction
Bombyx mori have been the best ‘model organism’ in Lepidoptera
and allowed researchers to make remarkable discoveries. For example,
Toyama (1906) confirmed Mendel’s laws of heredity is valid for B.
mori and this was the first case which proved the validity of Mendel’s
laws for an animal species. When Beadle and Tatum proposed ‘one
gene–one enzyme hypothesis’ (1941), Kikkawa (1941) almost
simultaneously reached the similar concept by using egg colour mutants
of B. mori . There is no doubt that availability of hundreds of
mutant strains contributed to those discoveries. Since the whole genome
sequence of B. mori was determined (International Silkworm Genome
Consortium, 2008), the usability of B. mori as model species has
significantly increased. Whole genome sequences of other lepidopteran
species, such as Papilio polytes , Danaus plexippus ,Lymantria dispar and so on, are now available and some
considerable studies were conducted by making the best of the genome
information (Gu et al., 2019; Nishikawa et al., 2015; Zhang et al.,
2019). However, species mentioned above have few mutant strains and
forward genetic analysis on those species have not been conducted.
However, being a model organism does not necessarily mean being an
ordinary species in its own taxon. Domestication had deprived B.
mori of many traits which majority of lepidopteran species possess,
such as the larval integument colour, foraging ability, adult flight
ability and so on. It is obvious that B. mori is not suitable for
researches attempting to elucidate the genetic basis of these traits,
demonstrating that researches using B. mori does not always lead
to better understanding of lepidopteran insects.
One possible answer to the question ‘Which species is better thanB. mori in genetic research?’ is Samia ricini (Fig. 1).S. ricini , also known as ‘Eri silkmoth,’ one of the Saturniid
moth species which was originated in Assam, India, and has been
artificially transferred to many Asian countries and other regions.
Although this species has been domesticated for the purpose of silk
production, S. ricini still retains the traits that are lost inB. mori . S. ricini is a multivoltine species while
majority of saturniids are univoltine or bivoltine (Brahma, Swargiary,
& Dutta, 2015; Sternburg, & Waldbauer, 1984), which means that
research of S. ricini is free from seasonal limitation (Singh,
Kumar, Ahmed, & Pathania, 2017). Also, S. ricini grows uniformly
and can be reared synchronously in large scale, resulting in efficient
egg production. Thus, we have already succeeded in establishing a
genome-editing system in this species using Transcription activator-like
effector nucleases (TALENs) and successfully obtained several gene
knockout lines (Lee, Kiuchi, Kawamoto, Shimada, & Katsuma, 2018),
meaning that functional analysis of genes of interest is now achievable.
Utilising S. ricini for genetic research has another advantage.
It will enable us to access to substantially high genetic diversity.
First of all, S. ricini reportedly consists of at least
twenty-six morphologically different eco races (Singh et al., 2017). In
addition, S. ricini is able to produce fertile hybrids with wildSamia species (Brahma et al., 2015; Peigler, & Naumann, 2003),
such as Samia canningi or Samia cynthia pryeri . Because
the populations of S. canningi and S. c. pryeri are
distributed throughout south and east Asian countries, different endemic
nature such as larval integument colour, larval marking patterns, cocoon
colour and host plant preference can be observed among populations
(Brahma et al., 2015; Peigler, & Naumann, 2003). Genetic diversity in
genus Samia rivals or even exceeds that in B. mori .
In order to facilitate genetic research of S. ricini , we decided
to determine the whole genome sequence of S. ricini . We employed
both long-read and short-read sequencers, namely Pacbio Sequel system
and illumina HiSeq1500 to construct high-quality genome assembly. After
the assembly was completed, to confirm whether genetic research aiming
at identifying trait-related genes is feasible or not, we attempted to
identify the responsible chromosomes for several phenotypes in S.
ricini and S. c. pryeri .