Re-elevation to species level and redescription of Serranochromis jallae and Serranochromis robustus (Teleostei: Cichlidae)

Serranochromis robustus robustus from Lake Malaŵi and Serranochromis robustus jallae from Zambia were compared using morphological data. We re-elevated S. robustus jallae to species based on the following. Serranochromis robustus generally has a longer lower jaw (50.7–59.6% HL) than S. jallae (49.2–52.7% HL). Serranochromis robustus is not as deep-bodied as S. jallae as evidenced by the distance between the posterior insertion of the dorsal fin and the posterior insertion of the anal fin (13.4–15.1% SL in S. robustus vs. 14.9–18.4% SL in S. jallae). Additionally, S. robustus has a narrower least caudal peduncle depth (10.9–12.8% SL) than S. jallae (11.3–14.2% SL); the least caudal peduncle depth of all S. robustus was less than 12.8% SL while, except for the smallest specimen of S. jallae (88.1 mm SL), the least caudal peduncle depth was greater than 13.2% SL. Adults in breeding color of Serranochromis robustus are blue/green laterally, while adults in breeding color of S. jallae are yellow/green laterally. The marginal bands on the dorsal and caudal fins of S. jallae are bright orange in specimens from the Okavango River system and creamy yellow in Upper Zambezi specimens. In S. robustus, there is a yellow marginal band on the dorsal fin.

Polyclad fauna of Agatti Island, Lakshadweep, India: new records and description of two new species

The present paper deals with the first study of polyclad fauna from Agatti Island, Lakshadweep. Through this study, Pseudobiceors apricus, Pseudobiceros hymanae and Pseudoceros bolool are recorded for the first time from Lakshadweep waters while Pseudoceros bicolor is recorded for the first time from Indian waters. Description of two new species, Pseudoceros agattiensis sp. nov. and Pseuodoceros stellans sp. nov. is also provided with good quality photographs of external and internal details. Pseudoceros agattiensis sp. nov. is characterized by a brown to black background colour, with minute white spots, marginal band thick and black with minute white spots present at very edge of the margin, three dorsal longitudinal white bands, out of which, two are laterally branched. Pseudoceros stellans sp. nov. displays a brown background colour with a black marginal band and completely covered with lighter microdots and a unique pattern of white dorsal blotches. This study adds to the knowledge of Indian polyclads, which are still an understudied group from Indian waters.

An essential role for α4A-tubulin in platelet biogenesis

During platelet biogenesis, microtubules (MTs) are arranged into submembranous structures (the marginal band) that encircle the cell in a single plane. This unique MT array has no equivalent in any other mammalian cell, and the mechanisms responsible for this particular mode of assembly are not fully understood. One possibility is that platelet MTs are composed of a particular set of tubulin isotypes that carry specific posttranslational modifications. Although β1-tubulin is known to be essential, no equivalent roles of α-tubulin isotypes in platelet formation or function have so far been reported. Here, we identify α4A-tubulin as a predominant α-tubulin isotype in platelets. Similar to β1-tubulin, α4A-tubulin expression is up-regulated during the late stages of megakaryocyte differentiation. Missense mutations in the α4A-tubulin gene cause macrothrombocytopenia in mice and humans. Defects in α4A-tubulin lead to changes in tubulin tyrosination status of the platelet tubulin pool. Ultrastructural defects include reduced numbers and misarranged MT coils in the platelet marginal band. We further observed defects in megakaryocyte maturation and proplatelet formation inTuba4a-mutant mice. We have, thus, discovered an α-tubulin isotype with specific and essential roles in platelet biogenesis.

Cytoskeleton mechanics determine resting size and activation dynamics of platelets

SummaryPlatelets are cell fragments of various size that help maintain hemostasis. The way platelets respond during a clotting process is known to depend on their size, with important physiological consequences. We characterized the cytoskeleton of platelets as a function of their size. In resting Human and Mice platelets, we find a quadradic law between the size of a platelet and the amount of microtubule polymer it contains. We further estimate the length and number of microtubules in the marginal band using Electron and Super-resolution microscopy. In platelets activated with ADP, the marginal band coils as a consequence of cortical contraction driven by actin. We observe that this elastic coiling response is accompanied by a reversible shortening of the marginal band. Moreover, larger platelets have a higher propensity to coil. These results establish the dynamic equilibrium that is responsible for platelet size and differential response on a more quantitative level.HighlightsPlatelet size scales consistently with amount of polymerized tubulin in both mouse and human.Polymerized actin is required for ADP-induced marginal band coiling.Upon activation, the marginal band exhibits a reversible visco-elastic response involving shortening.Larger marginal bands have a higher propensity to coil than shorter ones.In briefThe cytoskeleton is adapted to platelet size and its mechanical properties determine propensity of a platelet to undergo morphological changes in response to agonists.

Balance of microtubule stiffness and cortical tension determines the size of blood cells with marginal band across species

The fast bloodstream of animals is associated with large shear stresses. To withstand these conditions, blood cells have evolved a special morphology and a specific internal architecture to maintain their integrity over several weeks. For instance, nonmammalian red blood cells, mammalian erythroblasts, and platelets have a peripheral ring of microtubules, called the marginal band, that flattens the overall cell morphology by pushing on the cell cortex. In this work, we model how the shape of these cells stems from the balance between marginal band rigidity and cortical tension. We predict that the diameter of the cell scales with the total microtubule polymer and verify the predicted law across a wide range of species. Our analysis also shows that the combination of the marginal band rigidity and cortical tension increases the ability of the cell to withstand forces without deformation. Finally, we model the marginal band coiling that occurs during the disk-to-sphere transition observed, for instance, at the onset of blood platelet activation. We show that when cortical tension increases faster than cross-linkers can unbind, the marginal band will coil, whereas if the tension increases more slowly, the marginal band may shorten as microtubules slide relative to each other.

Selected Expression and Functional Importance of α4a-Tubulin in Platelet Biogenesis

Platelets are produced from mature megakaryocytes (MK) following a profound cellular reorganization. This includes the assembly of microtubules (MT) into a unique submenbranous coiled structure, the marginal band (MB). This process is thought to depend on a specific αβ-tubulin isotype repertoire. The MK-restricted-β1-tubulin, the predominant isoform of the MB, is already known to be important for platelet biogenesis but the implication of other isotypes is currently unknown. Our goal was to establish the αβ-tubulin repertoire in platelets and during megakaryopoiesis and to evaluate the implication of selected isotypes in platelet formation. To establish an exhaustive list of the tubulin isotypes, we used combination of RT PCR and proteomic analyses to quantify the expression of each isotype in human platelets and in human MK differentiated in culture from CD34+ hematopoietic progenitors. Information gained on the hierarchical combination of tubulin isoforms in the course of platelet biogenesis has been extended at the functional level to investigate both their role in marginal band formation and platelet functions β6-, β5- and α1c-tubulin transcripts were already present in CD34+ cells and decreased during the final stages of megakaryopoiesis. On the other hand, β1-, α4A- and α8-tubulin transcripts were only observed later during MK differentiation and in platelets. Quantitative LC-SRM mass spectrometry confirmed the predominant expression of β1 and α4A-isotypes in platelets. A functional role of the newly identified α4a-tubulin was supported by the thrombocytopenia and enlarged platelets with a decreased number of MT coils (1-3) comprising less-acetylated tubulin in mice carrying a point mutation in tuba4a. Additionally, a tendency to increased responses to several agonists was observed in these platelets. This study reveals new information on the evolution of the tubulin isotype repertoire in platelet formation pointing to a role of less-widely expressed α-isotypes. Disclosures No relevant conflicts of interest to declare.

Balance of microtubule stiffness and cortical tension determines the size of blood cells with marginal band across species

The fast blood stream of animals is associated with large shear stresses. Consequently, blood cells have evolved a special morphology and a specific internal architecture allowing them to maintain their integrity over several weeks. For instance, non-mammalian red blood cells, mammalian erythroblasts and platelets have a peripheral ring of microtubules, called the marginal band, that flattens the overall cell morphology by pushing on the cell cortex. In this article, we model how the shape of these cells stems from the balance between marginal band elasticity and cortical tension. We predict that the diameter of the cell scales with the total microtubule polymer, and verify the predicted law across a wide range of species. Our analysis also shows that the combination of the marginal band rigidity and cortical tension increases the ability of the cell to withstand forces without deformation. Finally, we model the marginal band coiling that occurs during the disc-to-sphere transition observed for instance at the onset of blood platelet activation. We show that when cortical tension increases faster than crosslinkers can unbind, the marginal band will coil, whereas if the tension increases slower, the marginal band may shorten as microtubules slide relative to each other.Significance StatementMany blood cells have a discoidal shape, which is essential for them to function properly within the organism. For some cells, such as blood platelets, this shape is due to the interplay between the elasticity of the marginal band, which is a closed ring of stiff filaments called microtubules, and the tension of the cell cortex, a polymer scaffold associated with the plasma membrane. Dmitrieff et al. examined how cell size is determined by the mechanical balance between these two components. Remarkably, the theory is confirmed over nearly three orders of magnitudes, by data collected from 25 species. The theory also shows how the composite structure is adapted to resist transient mechanical challenges, as encountered in the blood stream.