ABSTRACT
Neurogenesis is achieved through a sequence of steps that include specification and differentiation of progenitors into mature neurons. Frequently, precursors migrate to distinct positions before terminal differentiation. The Slit-Robo pathway, formed by the secreted ligand Slit and its membrane bound receptor Robo, was first discovered as a regulator of axonal growth. However, today, it is accepted that this pathway can regulate different cellular processes even outside the nervous system. Since most of the studies performed in the nervous system have been focused on axonal and dendritic growth, it is less clear how versatile is this signaling pathway in the developing nervous system. Here we describe the participation of the Slit-Robo pathway in the development of motion sensitive neurons of the Drosophila visual system. We show that Slit and Robo receptors are expressed in different stages during the neurogenesis of motion sensitive neurons. Furthermore, we find that Slit and Robo regulate multiple aspects of their development including neuronal precursor migration, cell segregation between neural stem cells and daughter cells and formation of their connectivity pattern. Specifically, loss of function of slit or robo receptors in differentiated motion sensitive neurons impairs dendritic targeting, while knocking down robo receptors in migratory progenitors or neural stem cells leads to structural defects in the adult optic lobe neuropil, caused by migration and cell segregation defects during larval development. Thus, our work reveals the co-option of the Slit-Robo signaling pathway in distinct developmental stages of a neural lineage.
ABSTRACT
Both the records and the descriptions of the crustose species of coralline algae on the southeastern coast of South America are from the early 1900s. Unlike other algae species on the coast of Chile, the biogeography and distribution of crustose corallines have not been studied despite their abundance. Through recent studies, it has been determined that the genus Spongites is the most conspicuous genus along the rocky intertidal of the Chilean coasts. It is also common to the entire coast of the Southern Hemisphere; however, the relationship between species and the possible reasons for their distribution is unknown. We used nuclear and mitochondrial genetic markers and SEM observations of morphological characters to examine Spongites samples from the Southern Hemisphere and to establish the phylogeographic relationships of Chilean Spongites with specimens from other southern coasts. The combination of these analyses revealed the following: (i) a monophyletic clade that represents the Chilean Spongites and (ii) a paraphyletic clade for South African, New Zealand, and Argentine samples. Consequently, we postulate two nonexclusive hypotheses regarding the relationship of Spongites species in the Southern Hemisphere: (i) a complex history of extinction, speciation, and recolonization that might have erased original Gondwanan split patterns, and (ii) an Antarctic Peninsula origin for the Chilean Spongites species.
ABSTRACT
Knowledge on species of the order Corallinales along the coast of Chile is still scarce despite a number of studies and records of other divisions of seaweeds made since the early 20th century. This lack of information is more dramatic among crustose representatives of the order, thus depriving biogeographic studies of a thorough analysis and resulting in inadequately representative accounts of biodiversity. The currently changing taxonomy of the group makes it difficult to identify and differentiate among taxa based on morphological and developmental characters. Therefore, the use of molecular tools has been adopted in this study in order to facilitate identification and comparison of crustose corallines collected at the rocky intertidal between 27 degrees and 48 degrees S along the Pacific temperate coast of South America. A sequence 600bp (in length) from the SSU-rDNA gene was used to identify five taxa to the genus level: Lithophyllum, Spongites, Mesophyllum, Synarthrophyton, and Leptophytum. In all cases, the genus distinction based on morphological characters coincide with designations based on variation in the ribosomal DNA gene sequence. Spongites is the most frequently occurring genus and is found in all localities sampled while the others appear occasionally. Taxa recognition at species level must be examined with caution considering that morphological variability is not well understood in Chile because the SSU-rDNA region sequence does not always stand alone as an unambiguous means of identifying all coralline species. In such cases, more rapidly evolving markers are needed. For example, sequences from the ITS (rDNA) region often provide greater resolution among closely related species and genera. However, the methodology presented here remains a useful tool for species-level identification.
