Formation of the germ-disc in spider embryos by a condensation-like mechanism

Background: 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. 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. results: here 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. conclusions: the 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. © 2016 the author(s).