Transport of Carotenoids by a Carotenoid-Binding Protein in the Silkworm - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 516

Symbol of gene
Acp
C
F
I
Pk
Y
Ymc

Locus
?
12-7.2
6-34.7
9-16.2
2-?
2-28.6
?

Phenotype
Light yellow hemolymph, even though individuals have Y and I
Making yellow cocoon in combination with Y and + I
Making yellowish flesh cocoon in combination with Y and + I
Colorless hemolymph, even though individuals have Y
Making pink cocoon in combination with Y , + I and F
Yellow hemolymph
Making light yellow cocoon in combination with Y A , an allele of Y

(a)

Midgut lumen Intestinal mucosal cell Hemolymph Silk gland cell

+ I

Y Y C

+ Y Y+ C I

(b)

FIGURE 24.4

I indi-
cates a recessive allele of I . (b) Schematic illustration of the function of the Y , I , and C genes. Only larvae with
the genotype [ Y +

Silkworm genetic loci responsible for carotenoid transport. (a) List of the genetic loci. +

I C ] transport carotenoids into the silk gland and create yellow cocoons.

carotenoids, resulting in colorless (carotenoid-dei cient) hemolymph. The I gene (Toyama 1912)
controls the transfer of carotenoids from the intestinal mucosal cells into the hemolymph (Nakajima
1963). Larvae of mutants with the dominant I allele are incompetent at transferring carotenoids
from intestinal mucosal cells, resulting in the accumulation of carotenoids in the intestinal mucosa.
A larval hemolymph of the I allele strain is therefore colorless even if the strain bears the Y allele.
The C gene (Uda 1919) controls the uptake of carotenoids, especially xanthophylls, from the hemo-
lymph into the cells in the middle part of the middle division of the silk gland. In contrast to the Y
gene, the C gene does not affect the uptake of carotenoids from the midgut lumen into the midgut
epithelium (Nakajima 1963). Only larvae with the genotype [ Y

+

I C ] create yellow cocoons. All

other combinations make white cocoons.

24.3.2 CBP I S A P RODUCT OF THE Y ELLOW B LOOD G ENE
Western blotting and immunohistochemistry clearly demonstrated that CBP is expressed in the
larvae of the Y allele strain and is not expressed in the larvae of the

Y allele strain (Figure 24.5a)
(Tabunoki et al. 2002, Tsuchida et al. 2004a). The genotype with respect to I and C genes had
no relationship with the level of the expression of CBP (Tabunoki et al. 2002, Tsuchida et al.
2004a). Restriction fragment length polymorphism (RFLP) mapping with a cDNA probe of the
CBP gene revealed that the CBP gene was on the second chromosome, which is the same as the
Y gene and different from the I (the 9th chromosome) and the C (the 12th chromosome) genes
(Hara et al. 2007). As indicated above, CBP is expressed in both the midgut and silk gland,
where the Y gene functions. These observations suggested that the Y gene corresponds directly
to the CBP gene.
Next, the difference in the genomic and cDNA sequences of CBP between the Y and

+

Y alleles
were determined (Figure 24.5b) (Sakudoh et al. 2005, 2007). The Y allele strain had at least two
copies of the CBP gene, all of which could be classii ed into one of two types: a Y -a sequence

+