Abstract
BackgroundDetermination of the embryonic body axes is a crucial developmental process in all animals. The establishment of the embryonic axes of spiders has been best studied in the common-house-spider Parasteatoda tepidariorum. Here, anteroposterior (AP) polarity arises during germ disc formation; the centre of the germ-disc marks the future posterior pole, and the rim of the disc the future anterior pole of the spider embryo. The centre of the germ disc is also needed for the formation of the cumulus, a group of migratory cells needed to establish dorsoventral (DV) polarity. Thus, both body axes depend on proper germ disc formation and patterning. However, these processes have not been fully analysed at the cellular and molecular level.ResultsHere I present new techniques to stain the cell membranes/outlines in live and fixed spider embryos. I show that the germ-disc is formed from a regular and contiguous blastoderm and that co-ordinated cell shape changes, rather than migration of single cells, are required to drive germ-disc formation in P. tepidariorum embryos. Furthermore, I show that the rate of cell divisions within the embryonic and extra-embryonic region is not involved in the rapid establishment of the germ-disc. Finally, I show that the process of germ-disc formation is dependent on the initiation of zygotic transcription.ConclusionsThe presented data provide new insights in to the formation of the germ-disc in spider embryos. The establishment of the germ-disc in Parasteatoda embryos is a highly dynamic process that involves wide scale cell-shape changes. While most of the blastodermal cells become cuboidal to form the dense germ-disc, the remaining blastodermal cells stay squamous and develop into huge extra-embryonic, yolk rich cells. In addition, this study shows that the onset of zygotic transcription is needed to establish the germ-disc itself, and that the mid-blastula transition of Parasteatoda tepidariorum embryos is prior to any overt axis establishment.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0166-9) contains supplementary material, which is available to authorized users.
Highlights
Determination of the embryonic body axes is a crucial developmental process in all animals
Spiders are basally branching arthropods [4, 5], and during the past 15 years the spider, Parasteatoda tepidariorum, has become a popular organism to study the evolution of developmental processes in arthropods [7, 8]
Germ-disc formation in P. tepidariorum: cell migration vs. cell shape changes What is the overall composition of the blastoderm shortly before germ-disc formation? Is a contiguous blastoderm present that requires cell shape changes to form the germdisc or does germ-disc formation require the migration of single cells? To answer these questions I carried out a search for markers to label the cell outline of P. tepidariorum embryos
Summary
Determination of the embryonic body axes is a crucial developmental process in all animals. The establishment of the embryonic axes of spiders has been best studied in the common-house-spider Parasteatoda tepidariorum. The centre of the germ-disc is needed for the formation of the cumulus, a group of migratory cells needed to establish dorsoventral (DV) polarity. Both body axes depend on proper germ-disc formation and patterning. These processes have not been fully analysed at the cellular and molecular level. Determination and patterning of the main body axes in a non-insect arthropod species, has been well analysed in the common house spider Parasteatoda tepidariorum. While several aspects of how the dorsoventral body axis is established in this organism have been revealed via time-lapse microscopy and gene knockdown experiments [9,10,11], only
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