PATTERNS IN THE INTEGUMENT OF INSECTS
77
If this observation should prove to be generally applicable, there may
be fairly precise parallels between induction as defined for vertebrate
differentiation (Fig. 10A) and some features of the gradient in insects
(Fig. 10B).
There are several parallels between the properties of the gradient described here and the concept of a "prepattern" (Stern, 1956; Maynard
Smith and Sondhi, 1961). Stern used genetic mosaics to analyse a number
of bristle patterns in Drosophila. In the mutant achaete, the posterior
pair of dorsocentral bristles on the thorax are absent. He found that no
bristle was formed in a predominantly wild-type fly if genetically
achaete tissue covered the site of the bristle, but that a normal bristle
arose when the site was covered by genetically wild-type tissue. He supposed that the presence of a bristle required the existence of a "prepattern" determining its position and orientation. The mutant achaete
only affects the competence of the epidermal cells to differentiate into
bristles and not the underlying "prepattern." The term "prepattern" is
perhaps unfortunate since it readily lends itself to mysticism. It is not
difficult to think of the epidermis having properties which determine the
position and orientation of bristles, and that these properties are determined genetically, independently of the competence to form bristles. We
can perhaps make a parallel between the dorsal abdominal cuticle of
adult Rhodnius and the achaete mutant of Drosophila. Neither has
bristles, but the epidermis in both is believed to have properties which
would influence the position and orientation of bristles if they were to be
there. Wild-type Drosophila would be paralleled by Rhodnius appendages
or sternites, both have a "prepattern" and bristles. Mutations affecting
these underlying properties, "prepattern" or gradient, might be more
interesting than those affecting the presence or absence of bristles.
8. Diffusible Factors in the Epidermis
The possibility that a diffusible factor may be involved in some interchanges between cells has been investigated in Rhodnius (Locke, 1966c).
The cell behavior involved in the migration of cells in the repair of an
excised region has two components, movement and its initiation, and direction. In an attempt to answer the question whether the stimulus to
migrate is due to the removal of normal communication with other cells,
a procedure was developed whereby the epidermis at the front of a segment was separated from that at the back by a strip of test material
(Fig. 40). It was found that the epidermis migrated over inert impermeable strips, but did not do so over millipore filters. The failure to migrate
could be correlated with (1) the porosity of the filter, and (2) the fact
that epidermal cells were on each side of these pores. It was probably not
due to the nature of the material composing the filter, since the epidermis
77
If this observation should prove to be generally applicable, there may
be fairly precise parallels between induction as defined for vertebrate
differentiation (Fig. 10A) and some features of the gradient in insects
(Fig. 10B).
There are several parallels between the properties of the gradient described here and the concept of a "prepattern" (Stern, 1956; Maynard
Smith and Sondhi, 1961). Stern used genetic mosaics to analyse a number
of bristle patterns in Drosophila. In the mutant achaete, the posterior
pair of dorsocentral bristles on the thorax are absent. He found that no
bristle was formed in a predominantly wild-type fly if genetically
achaete tissue covered the site of the bristle, but that a normal bristle
arose when the site was covered by genetically wild-type tissue. He supposed that the presence of a bristle required the existence of a "prepattern" determining its position and orientation. The mutant achaete
only affects the competence of the epidermal cells to differentiate into
bristles and not the underlying "prepattern." The term "prepattern" is
perhaps unfortunate since it readily lends itself to mysticism. It is not
difficult to think of the epidermis having properties which determine the
position and orientation of bristles, and that these properties are determined genetically, independently of the competence to form bristles. We
can perhaps make a parallel between the dorsal abdominal cuticle of
adult Rhodnius and the achaete mutant of Drosophila. Neither has
bristles, but the epidermis in both is believed to have properties which
would influence the position and orientation of bristles if they were to be
there. Wild-type Drosophila would be paralleled by Rhodnius appendages
or sternites, both have a "prepattern" and bristles. Mutations affecting
these underlying properties, "prepattern" or gradient, might be more
interesting than those affecting the presence or absence of bristles.
8. Diffusible Factors in the Epidermis
The possibility that a diffusible factor may be involved in some interchanges between cells has been investigated in Rhodnius (Locke, 1966c).
The cell behavior involved in the migration of cells in the repair of an
excised region has two components, movement and its initiation, and direction. In an attempt to answer the question whether the stimulus to
migrate is due to the removal of normal communication with other cells,
a procedure was developed whereby the epidermis at the front of a segment was separated from that at the back by a strip of test material
(Fig. 40). It was found that the epidermis migrated over inert impermeable strips, but did not do so over millipore filters. The failure to migrate
could be correlated with (1) the porosity of the filter, and (2) the fact
that epidermal cells were on each side of these pores. It was probably not
due to the nature of the material composing the filter, since the epidermis