Embryogenesis

Distinct patterns of embryonic development are observed in hemimetabolous and holometabolous insects. In both types, the embryo produces multiple cuticular layers, and the appearance of these coincides with pulses of ecdysteroids. Juvenile hormone levels are generally low during early embryogenesis, but climb later to program nymphal or larval cuticle formation upon appearance of an ecdysteroid peak.

In the hemimetabolous grasshopper Locusta, four peaks of ecdysteroids are observed, corresponding to production of serosal cuticle and three embryonic cuticle layers (Fig. 1A). JH levels are elevated immediately after oviposition, because of maternal contribution to the yolk, but rapidly decrease to

% of Embryonic Development % of Embryonic Develop men I

FIGURE 1 Hormone levels during embryonic molts in a hemimetabolous insect, the grasshopper Locusta migratoria (A), and in a holometabolous insect, the moth, Manduca sexta (B). During Locusta embryogenesis, four peaks of ecdysteroid are observed, each corresponding to secretion of a layer of cuticle. The first peak of predominantly aE at 20% development initiates secretion of the serosal cuticle (not shown). A second peak prior to protho-racic gland differentiation (PG) causes secretion of the first embryonic cuticle (E1). Just after blastokinesis (Bla), a third ecdysteroid peak leads to secretion of the pronymph cuticle. Shading of the pronymph time line at the top of the plot indicates the pharate stage, which ends with ecdysis of the E1 cuticle (vertical dotted line). The fourth ecdysteroid peak, occurring for the first time in the presence of JH, contains approximately equal amounts of aE, 20HE, and 20,26HE. This causes secretion of the first-stage nymphal cuticle. At hatching the nymph sheds the pronymph cuticle upon escaping from the substrate. Secretion of the first embryonic cuticle in Manduca occurs in the absence of an ecdysteroid peak. The first larval cuticle is secreted in response to elevated ecdysteroids in the presence of JH just after dorsal closure (DC). Note that JH levels in Manduca rise earlier in development than in Locusta and that ecdysteroid signaling always occurs in the presence of JH. (Adapted from Truman and Riddiford, 1999, 2002.)

low levels. At —20% of embryonic development and before prothoracic glands are developed, the first ecdysteroid peak consisting exclusively of aE occurs in the presence of relatively low JH levels. This first peak comes from the release of maternal ecdysteroids stored as polar conjugates, and shortly thereafter the serosal cuticle is secreted. A second aE peak occurs at —30% development, leading to formation of the first embryonic cuticle. A third ecdysteroid peak occurs just after differentiation of the prothoracic glands, and this coincides with the first appearance of 20HE and 20,26HE. Nevertheless, levels of aE together with 20,26HE predominate at this time, leading to the secretion of the second embryonic cuticle, which Truman and Riddiford refer to as pronymphal cuticle. It is the first layer of cuticle tough enough to require shedding via ecdysis behavior. The coincidence of the early, aE peaks with cuticle secretion suggests that aE is not only a 20HE precursor, but also a biologically active hormone at certain times of development. At 70% development, the first embryonic cuticle is shed and followed quickly by a large peak of ecdysteroids. This is the first exposure of the embryo to substantial 20HE levels in the presence of JH, leading to synthesis of the first instar nymphal cuticle. This peak of ecdysteroids contains large amounts of aE, 20HE, and 20,26HE.

At hatching, the grasshopper emerges from the egg under the ground, still surrounded by the pronymphal cuticle. Despite its hexapod body plan, the animal exhibits a classic vermiform (wormlike) locomotory pattern as it escapes the egg pod and maneuvers through the substrate to the surface. In a matter of seconds to minutes, the pronymphal cuticle is shed, and the animal stretches its legs and switches abruptly to hexapod behavior. Truman and Riddiford have called attention to many similarities between the hemimetabolous pronymph and the holometabolous larva, suggesting that the latter has resulted from a hormonal shift in embryogenesis, resulting in an extended postembryonic phase of pronymph development. The ancestral pronymph undergoes an extended, multistage developmental sequence as a larva in the Holometabola.

The importance of a JH-free period during early embryogenesis of hemimetabolous insects (grasshopper—Schistocerca, cricket—Acheta) has been demonstrated by treatment of eggs with JH analogs. This results in inhibition of blastokinesis, reduction in the number of embryonic cuticle layers produced, premature appearance of nymphal cuticle and mouthparts, and reduced body size.

Embryogenesis in holometabolous Lepidoptera is somewhat simpler, with the secretion of only three cuticles, one serosal and two embryonic cuticles. Levels of both ecdysteroids and JH are undetectable early in embryogenesis, but rise earlier compared with the Hemimetabola, or at about 30% development (Fig. 1B), preceding the ecdysteroid peak. Therefore, unlike hemimetabolous embryogenesis, the first exposure to ecdysteroids occurs in the presence of JH, leading to production of the first larval cuticle. An

FIGURE 2 Hormonal regulation of development in postembryonic stages of the cockroach Nauphoeta cinerea, representing hemimetabolous development. During the immature stages of Nauphoeta, JH levels are elevated each time ecdysteroid levels rise to initiate a molt, resulting in secretion of nymphal cuticle. Following ecdysis to the last instar nymph, JH levels drop precipitously. Adult commitment is signaled by a biphasic ecdysteroid peak in the absence of JH. Incomplete metamorphosis occurs during the period between ecdysteroid elevation and ecdysis to the adult stage. (Adapted from Lanzrein et al., 1985.)

FIGURE 2 Hormonal regulation of development in postembryonic stages of the cockroach Nauphoeta cinerea, representing hemimetabolous development. During the immature stages of Nauphoeta, JH levels are elevated each time ecdysteroid levels rise to initiate a molt, resulting in secretion of nymphal cuticle. Following ecdysis to the last instar nymph, JH levels drop precipitously. Adult commitment is signaled by a biphasic ecdysteroid peak in the absence of JH. Incomplete metamorphosis occurs during the period between ecdysteroid elevation and ecdysis to the adult stage. (Adapted from Lanzrein et al., 1985.)

embryonic ecdysis occurs at 70% development, and first instar larval hatching does not involve cuticle shedding. The importance of a JH-free period observed for hemimetabolous insects is not the case for embryogenesis in the Holo-metabola. Embryos are largely insensitive to exogenous JH treatment, suggesting perhaps the absence of receptors for these hormones until later in embryogenesis.

Bee Keeping

Bee Keeping

Make money with honey How to be a Beekeeper. Beekeeping can be a fascinating hobby or you can turn it into a lucrative business. The choice is yours. You need to know some basics to help you get started. The equipment needed to be a beekeeper. Where can you find the equipment you need? The best location for the hives. You can't just put bees in any spot. What needs to be considered when picking the location for your bees?

Get My Free Ebook


Post a comment