A revised understanding of Tribolium morphogenesis further reconciles short and long germ development

Abstract

In Drosophila melanogaster, the germband forms directly on the egg surface and solely consists of embryonic tissue. In contrast, most insect embryos undergo a complicated set of tissue rearrangements to generate a condensed, multilayered germband. The ventral side of the germband is embryonic, while the dorsal side is thought to be an extraembryonic tissue called the amnion. While this tissue organisation has been accepted for decades and has been widely reported in insects, its accuracy has not been directly tested in any species. Using live cell tracking and differential cell labelling in the short germ beetle Tribolium castaneum, I show that most of the cells previously thought to be amnion actually give rise to large parts of the embryo. This process occurs via the dorsal-to-ventral flow of cells and contributes to germband extension (GBE). In addition, I show that true ‘amnion’ cells in Tribolium originate from a small region of the blastoderm. Together, my findings show that development in the short germ embryos of Tribolium and the long germ embryos of Drosophila is more similar than previously proposed. Dorsal-to-ventral cell flow also occurs in Drosophila during GBE, and I argue that the flow is driven by a conserved set of underlying morphogenetic events in both species. Furthermore, the revised Tribolium fate map that I present is far more similar to that of Drosophila than the classic Tribolium fate map. Lastly, my findings show that there is no qualitative difference between the tissue structure of the cellularised blastoderm and the short/intermediate germ germband. As such, the same tissue patterning mechanisms could function continuously throughout the cellularised blastoderm and germband stages, and easily shift between them over evolutionary time.

Arthropod segmentation

“…In the beetle Tribolium, split-embryo experiments have confirmed that this variability results from a temporally dynamic 'segmentation clock' within individuals rather than spatially variable expression between individuals (Sarrazin et al, 2012). Expression dynamicity has also been demonstrated in Tribolium by comparing the average patterns of finely staged cohorts of embryos, by visualising discrepancies between the transcript and protein domains of a given gene, and by gaining an understanding of cell dynamics within the SAZ via live imaging (Benton, 2018;El-Sherif et al, 2012;Sarrazin et al, 2012). In other species, gene expression dynamics within the SAZ have rarely been studied in detail.…”

“…'SAZ' is now preferred over the traditional term 'growth zone', because it makes no assumption of localised and continuous cell proliferation in the posterior of the embryo (Janssen et al, 2010). The material for new segments is generally provided by a combination of cell division and convergent extension, butas in vertebratesthe relative contributions of these cell behaviours to axial elongation vary widely across species (Auman et al, 2017;Benton, 2018;Benton et al, 2016;Mito et al, 2011;Nakamoto et al, 2015;Steventon et al, 2016). Accordingly, although cell division may in some species be coordinated with segment addition, segment patterning processes do not appear to be mechanistically dependent on the cell cycle (Cepeda et al, 2017), aside from in special cases such as malacostracan crustaceans.…”

Multiple Roles of the Polycistronic Gene Tarsal-less/Mille-Pattes/Polished-Rice During Embryogenesis of the Kissing Bug Rhodnius prolixus

“…Since ectopic segment formation after mlpt RNAi occurs in a low frequency in R. prolixus, it is possible that it also occurs at low frequency in O. fasciatus and G. buenoi mlpt knockdown. It is intriguing that, in these three species, mlpt is mainly required when germ band elongation and abdominal segmentation takes place and future studies using embryonic live imaging, as recently described for other insect species (Benton, 2018), might help to understand the role of mlpt during germ band elongation and thoracic vs. abdominal segment specification.…”

Elongation during segmentation shows axial variability, low mitotic rates, and synchronized cell cycle domains in the crustacean, Thamnocephalus platyurus

“…Segments are added at an average rate slightly less than one segment per hour at 30 °C (0.7 segments/h or 1.4 h per segment). The regularity of segment addition is unaffected by either the first molt (~ 4 h post-hatching, see Additional file 2 for how first molt was determined) or the transitions between addition of thoracic (post-maxillary segments, 1-11), genital (12,13), and abdominal segments (14-19, Additional file 1). Within 18 h at 30 °C, larvae add 14 segments, and the overall length of the body roughly doubles ( Fig.…”

“…Because most species elongate significantly during segmentation, classical concepts of posterior growth generally invoke mitosis, either in posterior stem cells or in a vaguely defined posterior region of proliferation [4][5][6][7][8]. Cell movement has also been assumed to play a role in elongation in cases where embryonic shape changes dramatically [7][8][9][10]-and is documented in the flour beetle, Tribolium castaneum [11][12][13]. The current descriptive data suggest a large degree of variability in how sequentially segmenting arthropod embryos grow (reviewed in [7,14,15]).…”