No abstract

The bioassessment of aquatic ecosystems is currently based on various biotic indices that use the occurrence and/or abundance of selected taxonomic groups to define ecological status. These conventional indices have some limitations, often related to difficulties in morphological identification of bioindicator taxa. Recent development of DNA barcoding and metabarcoding could potentially alleviate some of these limitations, by using DNA sequences instead of morphology to identify organisms and to characterize a given ecosystem. In this paper, we review the structure of conventional biotic indices, and we present the results of pilot metabarcoding studies using environmental DNA to infer biotic indices. We discuss the main advantages and pitfalls of metabarcoding approaches to assess parameters such as richness, abundance, taxonomic composition and species ecological values, to be used for calculation of biotic indices. We present some future developments to fully exploit the potential of metabarcoding data and improve the accuracy and precision of their analysis. We also propose some recommendations for the future integration of DNA metabarcoding to routine biomonitoring programs.

The reproduction of Stylophora pistillata, one of the most abundant coral species in the Gulf of Eilat, Red Sea, was studied over more than two years. Gonads were regularly examined using histological sections and the planula-larvae were collected in situ with plankton nets. S. pistillata is an hermaphroditic species. Ovaries and testes are situated in the same polyp, scattered between and beneath the septa and attached to them by stalks. Egg development starts in July preceding the spermaria, which start to develop only in October. A description is given on the male and female gonads, their structure and developmental processes. During oogenesis most of the oocytes are absorbed and usually only one oocyte remains in each gonad. S. pistillata broods its eggs to the planula stage. Planulae are shed after sunset and during the night. After spawning, the planula swims actively and changes its shape frequently. A mature planula larva of S. pistillata has 6 pairs of complete mesenteries (Halcampoides stage). However, a wide variability in developmental stages exists in newly shed planulae. The oral pole of the planula shows green fluorescence. Unique organs ('filaments' and 'nodules') are found on the surface of the planula; these might serve as attachment organs. A hypothesis is offered for further examination suggesting a trend of brooding versus non-brooding species in scleractinian corals: Coral species which develop gonads in their body cavities (usually branching forms or small polyped corals) reduce the number of eggs during oogenesis, have small eggs and brood planula-larvae, while coral species which develop gonads within their mesenteries (usually massive growth forms or large polyped corals) have numerous and large ova per polyp and expel their eggs into the water.

Cell differentiation, tissue formation, and organogenesis are fundamental patterns during the development of multicellular animals from the dividing cells of fertilized eggs. Hence, the complete morphogenesis of any developing organism of the animal kingdom is based on a complex series of interactions that is always associated with the development of a blastula, a one-layered hollow sphere. Here we document an alternative pathway of differentiation, organogenesis, and morphogenesis occurring in an adult protochordate colonial organism. In this system, any minute fragment of peripheral blood vessel containing a limited number of blood cells isolated from Botrylloides, a colonial sea squirt, has the potential to give rise to a fully functional organism possessing all three embryonic layers. Regeneration probably results from a small number of totipotent stem cells circulating in the blood system. The developmental process starts from disorganized, chaotic masses of blood cells. At first an opaque cell mass is formed. Through intensive cell divisions, a hollow, blastula-like structure results, which may produce a whole organism within a short period of a week. This regenerative power of the protochordates may be compared with some of the characteristics associated with the formation of mammalian embryonal carcinomous bodies. It may also serve as an in vivo model system for studying morphogenesis and differentiation by shedding more light on the controversy of the "stem cell" vs. the "dedifferentiation" theories of regeneration and pattern formation.

Summary The mechanisms that sustain stem cells are fundamental to the maintenance of tissues/organs. Here we identify ‘cell-islands’ (CIs) as a niche for putative germ and somatic stem cells in Botryllus schlosseri, a colonial chordate that undergoes weekly cycles of death and regeneration. Cells within CIs express markers associated with germ and somatic stem cells and gene products that implicate CIs as signaling centers for stem cells. Transplantation of CIs induced long-term germ-line and somatic chimerism, demonstrating self-renewal and pluripotency of CI-cells. Cell labeling and in-vivo time-lapse imaging of CI-cells reveal waves of migrations from degrading CIs, into developing buds, contributing to soma and germ-line development. Knockdown of cadherin, which is highly expressed within CIs, elicited the migration of CI-cells to circulation. Piwi-knockdown resulted in regeneration arrest. We suggest that repeated trafficking of stem cells allow them to escape the constraints imposed by the niche, thereby promoting their self-preservation throughout life.