Interrelationships between developing oocytes and ovarian tissues in the chiton Sypharochiton septentriones (Ashby) (Mollusca, Polyplacophora)

Filling a blank in bivalve taxonomy: an integrative analysis of Cyamioidea (Mollusca: Bivalvia)

This contribution provides the first integrative study of Cyamioidea, a group of bivalves largely neglected in the past. By combining information obtained from shell morphology, gross anatomy, histology, reproductive biology and DNA sequencing, the distinctive characters of Cyamioidea are determined, and the taxa belonging to this superfamily are revised. Gaimardioidea, previously considered as a distinct superfamily, is reduced to family level (Gaimardiidae), which, along with Cyamiidae, are assigned to Cyamioidea. All studied cyamioideans share a common reproductive character: the presence of a true follicular epithelium surrounding each developing oocyte, which persists after spawning, surrounding the developing embryos and participating in their anchorage to the gill filaments. Several morphological and anatomical characters support the distinction of Gaimardiidae and Cyamiidae. Based on that information, the placement of Gaimardia and Kidderia in Gaimardiidae, and that of Cyamiocardium, Cyamiomactra, Cyamium, Heteromactra, Pseudokelly, Ptychocardia and Reloncavia in Cyamiidae, is confirmed. Jukesena, historically placed in Veneroidea, is here allocated to Cyamiidae. Neoleptonidae, previously also attributed to Cyamioidea, are reallocated to Veneroidea.

Pursuing the quest for better understanding the taxonomic distribution of the system of doubly uniparental inheritance of mtDNA

There is only one exception to strict maternal inheritance of mitochondrial DNA (mtDNA) in the animal kingdom: a system named doubly uniparental inheritance (DUI), which is found in several bivalve species. Why and how such a radically different system of mitochondrial transmission evolved in bivalve remains obscure. Obtaining a more complete taxonomic distribution of DUI in the Bivalvia may help to better understand its origin and function. In this study we provide evidence for the presence of sex-linked heteroplasmy (thus the possible presence of DUI) in two bivalve species, i.e., the nuculanoid Yoldia hyperborea(Gould, 1841)and the veneroid Scrobicularia plana(Da Costa,1778), increasing the number of families in which DUI has been found by two. An update on the taxonomic distribution of DUI in the Bivalvia is also presented.

Emergence, development, and maturity of the gonad of two species of chitons "sea cockroach" (Mollusca: Polyplacophora) through the early life stages

This study describes and recognises, using histological and microscopical examinations on a morphometrical basis, several gonad traits through the early life stages of Chiton articulatus and C. albolineatus. Gonadal ontogenesis, gonad development stages, sexual differentiation, onset of the first sexual maturity, and growth sequences or "early life stages" were determined. In addition, allometry between lengths and body weight pooled for both sexes per each chiton were calculated using equation Y = aX(b) . A total of 125 chitons (4≤TL≤40 mm, in total length "TL") were used. All allometric relations showed a strong positive correlation (r), close to 1, with b-values above three, indicating an isometric growth. Gonadal ontogenesis and gonad development stages were categorised into three periods ("Pw" without gonad, "Pe" gonad emergence, and "Pf" gonadal sac formed) and four stages ("S0" gametocytogenesis, "S1" gametogenesis, "S2" mature, and "S3" spawning), respectively. Compound digital images were attained for each process. Periods and stages are overlapped among them and between species, with the following overall confidence intervals in TL: Pw 6.13-14.32 mm, Pe 10.32-16.93 mm, Pf 12.99-25.01 mm, S0 16.08-24.34 mm (females) and 19.51-26.60 mm (males), S1 27.15-35.63 mm (females) and 23.45-32.27 mm (males), S2 24.48-40.24 mm (females) and 25.45-32.87 mm (males). Sexual differentiation (in S0) of both chitons occurs first as a female then as a male; although, males reach the onset of the first sexual maturity earlier than females, thus for C. articulatus males at 17 mm and females at 32 mm, and for C. albolineatus males at 23.5 mm and females at 28 mm, all in TL. Four early life stages (i.e., subjuvenile, juvenile, subadult, and adult) are described and proposed to distinguish growth sequences. Our results may be useful to diverse disciplines, from developmental biology to fisheries management.

Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

Cell lineage in molluscan development

Cell lineage specification in molluscs is brought about by two mechanism: the segregation of morphogenetic plasms and inductive cell interactions. The evidence for the existence of morphogenetic plasms is largely circumstantial, but in one species, Bithynia, such a plasm has been identified in the polar lobe that forms at first cleavage. Inductive cell interactions are thought to be a prerequisite for the development of a large number of tissues and organs. The most extensively studied example is the specification of the mesodermal stem cell in Lymnaea and Patella, which occurs between 5th and 6th cleavage through an interaction between one macromere and a large number of micromeres. Both segregation and induction are tuned to the animal-vegetal polarity of the egg, at least during early development. This polarity probably arises during oogenesis and is manifest in regional differentiations of the surface architecture of the egg, in the distribution of inner membrane particles in the plasma membrane, in membrane fluidity characteristics, in ionic conductance properties of the plasma membrane, etc. All these phenomena have in common that they represent properties of the egg surface, suggesting that the polarity of the egg is somehow imprinted into the plasma membrane and the cortex of the egg during oogenesis.