Anterior and posterior centers jointly regulate Bombyx embryo body segmentation

Abstract

Insect embryo segmentation is largely divided into long and short germ types. In the long germ type, each segment primordium is represented on a large embryonic rudiment of the blastoderm, and segmental patterning occurs nearly simultaneously in the syncytium. In the short germ type, however, only anterior segments are represented in the small embryonic rudiment, usually located on the egg posterior, and the rest of the segments are added sequentially from the posterior growth zone in a cellular context. The long germ type is thought to have evolved from the short germ type. It is proposed that this transition, which appears to have occurred multiple times over the course of evolution, was realized through the acquisition of a localized anterior instruction center. Here, I examined the early segmentation process in the silkmoth Bombyx mori, a lepidopteran insect, in which the mechanisms of anterior-posterior (AP) axis formation have not been well analyzed. In this insect, both the long germ and short germ features have been reported. The mRNAs for two key genes involved in insect AP axis formation, orthodenticle (Bm-otd) and caudal (Bm-cad), are localized maternally in the germ anlage, where they act as anterior and posterior instruction centers, respectively. RNAi studies indicate that, while Bm-cad affects the formation of all the even skipped (Bm-eve) stripes, there is also anterior Bm-eve stripe formation activity that involves Bm-otd. Thus, there is redundancy in Bm-eve stripe formation activity that must be coordinated. Some genetic interactions, identified either experimentally or hypothetically, are also introduced, which might enable robust AP formation in this organism.

Parental RNA interference of genes involved in embryonic development of the western corn rootworm, Diabrotica virgifera virgifera LeConte

“…Since this early report, parental RNAi has been used to describe the function of embryonic genes in a number of other insect species including the milkweed bug, Oncopeltus fasciatus (Liu and Kaufman, 2004), the cricket, Gryllus bimaculatus (Mito et al, 2006), the springtail, Orchesella cincta (Konopova and Akam, 2014), the sawfly, Athalia rosae (Yoshiyama et al, 2013), the German cockroach, Blattella germanica (Piulachs et al, 2010), the silkworm, Bombix mori (Nakao, 2012), and the pea aphid, Acyrthosiphon pisum (Mao et al, 2013). The pRNAi response in all these instances was achieved by injection of dsRNA into the hemocoel of the parental female.…”

Analyses of interactions among pair-rule genes and the gap gene Krüppel in Bombyx segmentation

“…If ancestral segmentation mechanisms are like those proposed for Tribolium, this possibility implies that part of the selfregulatory mechanism(s) (possibly involving odd-related gene oscillation) is taken over by a hierarchical mechanism during the evolution of Bombyx, and this might represent additional diversity in insect segment mechanisms. However, additional complexities appear to exist in Bombyx segmentation mechanisms; mechanisms as hinted at in this pair-rule interaction study (possibilities of some stripe specific regulations) or those in the previous finding that the Bombyx orthodenticle homolog functions as an anterior factor regulating anterior Bm-eve stripe formation (Nakao, 2012), which suggests that Bombyx segmentation mechanisms operating in the anterior and posterior of the embryos are different (as in Nasonia).…”

“…Specifically, segregation of Bm-odd stripes from a pre-existing domain was perturbed, and Bm-odd stripe #2 and the adjacent domain were fused. This is reminiscent of the situation in Drosophila since the anterior and posterior boundary of the Bm-Kr domain lie between Bm-eve stripes #2 and #3, and stripes #3 and #4, respectively (Nakao, 2012); the segregation failure (or posterior expansion) of Bm-odd stripe #2 might be explained by a lack of (Xu et al, 1997), even-numbered Bm-en stripes overlap with Bm-odd stripes (see Fig. 6).…”

“…To examine how pair-rule gene expression is influenced by alterations in gap gene expression levels, an RNAi knockdown experiment was performed for Bm-Kr, the only known gap gene homolog in Bombyx (Nakao, 2012). Morphologically, Bm-Kr RNAi embryos exhibited large deletions (or fusion) in posterior segments (n ¼28/28).…”

“…All segments are fate mapped without a growth zone within a large germ anlage at the blastoderm stage, but the embryo exhibits sequential segmentation that progresses from anterior to posterior, at least as evidenced by the marker gene even-skipped (Bm-eve) (Myohara, 1994;Nakao, 2010Nakao, , 2012. Additionally, Bombyx embryogenesis essentially proceeds in the cellular environment; when energids enter the periplasm in AP progression, they soon cellularize (Takesue et al, 1980); thus Bombyx does not possess a syncytial blastoderm stage.…”