Shedding light on the embryogenesis and eye development of the troglophile cave spider Tegenaria pagana C. L. Koch, 1840 (Araneae: Agelenidae)

Background: Relatively little is known about the diversity of embryonic development across lineages of spiders, even though the study of embryonic development is a primary step in evo-devo studies and essential for understanding phenotypic evolution. Practically nothing is known about embryogenesis in cave-dwelling spiders, animals which play an important role in cave ecosystems and may have remarkable adaptations to aphotic habitats such as loss of eyes. Results: Here, we describe embryogenesis and study the expression patterns of several genes of the Retinal Determination Network (RDN) in the troglophile (species that have pre-adaptations to life in caves, and can complete their life cycle in caves, as well as in epigean habitats) eye-bearing funnel-web spider species Tegenaria pagana C. L. Koch, 1840, using fluorescent staining and confocal microscopy. We discuss the characteristic features of T. pagana embryogenesis and key RDN genes. Although in many respects the embryonic development of different species of entelegyne spiders is similar, we found differences in the rate of development, and the details of the opisthosoma, respiratory system, and brain morphogenesis in comparison with established spider model species. Our data supports the hypothesis of a conserved role of sine oculis gene in the eye formation of arachnids. Conclusions: Given the recent discovery of congeneric cave species with different degrees of eye reduction throughout Israel, these data sets provide a foundational point of comparison for studying eye reduction and eye loss events in the spider genus Tegenaria.