Sperm ultrastructure and spermiogenesis in two Exogone species (Polychaeta, Syllidae, Exogoninae)

The spermatozoa of Exogone naidina and E. dispar are characterized by a prominent bell-shaped acrosome, a spheroidal nucleus, and a conventional flagellum. During spermiogenesis, the acrosomal vesicle undergoes conspicuous modifications leading to its final bell shape with a posterior opening. The subacrosomal material initially shows radiating filaments but in mature sperms it appears as a meshwork of electron-opaque material. The acrosomal axis is oblique with respect to the main longitudinal sperm axis. The chromatin is arranged in electronopaque strands in the early spermatids, then becomes amorphous, and is finally organized in filaments in mature sperms. Centrioles are orthogonally arranged beneath the nucleus and fibers radiate from the distal centriole to contact the plasma membrane and the single mitochondrion. The latter is located eccentrically on the side of the nucleus opposite the acrosome. A disk-shaped structure is evident beneath the distal centriole. The flagellar axoneme has a 9+2 microtubule pattern. A conspicuous glycocalyx surrounds the flagellar plasma membrane, and an electron-lucent space is present between these two structures at the distal tip of the flagellum. We compare the sperm morphology of these two species of Exogone with that described in other members of the subfamily Exogoninae. The fine structure of these two species supports the occurrence of an ent-aquasperm type within Exogoninae, in accordance with the brood strategy present within this subfamily. The mode of reproduction is of taxonomic importance for defining subfamilies within Syllidae, and is likely also of phylogenetic significance. Because epitoky is probably plesiomorphic, the ent-aquasperm type found in Exogoninae can be considered a derived feature within Syllidae

Sperm ultra-structure of <i>Odontosyllis ctenostoma</i> (Polychaeta: Syllidae) with inferences on syllid phylogeny and reproductive biology

The analysis of the complex reproductive patterns of Syllidae may drastically change the taxonomic hierarchy of the family. To further contribute to the knowledge of Syllidae we have described the sperm ultra-structure and some steps of spermiogenesis of Odontosyllis ctenostoma Claparède, 1868, the first non interstitial eusylline investigated. The mature sperm has a bell-shaped acrosome and contains electron-dense granular material and thin filaments. The barrel-shaped nucleus bears two depressions: one anterior facing the acrosome and the other posterior partially containing the distal centriole and up to six mitochondria. Odontosyllis ctenostoma spermatozoa can be ascribed to the ect-aquasperm type typical of species practising external fertilisation. This morphology is not in complete accordance either with the particular brood protection reported for the species or the egg size. The sperm is similar to those of the Syllinae species thus far investigated, but the acrosome resembles that of the exogonine spermatid. Some authors consider Odontosyllis to be phylogenetically closer to Syllinae, though it shares epigamy with Exogoninae. Others have hypothesised that the exogonine sperm could have been derived from sylline sperm by simplification. In our hypothesis this could have happened through a gradual passage from an Odontosyllis-like eusylline ancestor

Sperm ultra-structure and spermiogenesis in <i>Syllis krohni</i> (Polychaeta: Syllidae), with some observations on its reproductive biology

yllidae show a wide spectrum of both reproductive strategies and sperm types. Analysis of their reproductive patterns could drastically change the presently accepted taxonomic hierarchy of the group. To further contribute to the knowledge of Syllidae we have described the sperm ultra-structure and the spermiogenesis of Syllis krohni (Ehlers, 1864). Mature sperm has a cone-shaped acrosome whose distal end is notched by a trough that transversely encircles its anterior portion. During spermiogenesis, the acrosomal vesicle undergoes modifications leading to its final shape with a posterior opening. The nucleus appears flattened anteriorly and forms a cleft that surrounds the proximal centriole in its distal region. A 9+2 flagellar axoneme is observed. Up to five mitochondria surround the distal centriole. The spermatozoon of S. krohni can be ascribed to the ect-aquasperm type that is typical in species having external fertilisation and appears to be very similar in appearance to that of the congeneric species thus far investigated. The main difference is in the shape of the acrosome, which is more elongated and as long as the nucleus. Reproduction of syllids seems to be conservative within sub-families, and the sperm morphology can probably help in phylogenetic reconstruction. External fertilisation is a widespread strategy within the genus Syllis, probably leading to a substantial similarity in sperm morphology being maintained. It is hypothesised, however, that within the same sperm type linked to a particularly reproductive strategy, the acrosome ultra-structure can be indicative of phylogeny

External gestation of Exogone naidina Öersted, 1845 (Polychaeta, Syllidae): Ventral attachment of eggs and embryos

The external gestation of sexually ripe females of the species Exogone naidina (Polychaeta, Syllidae) is described by means of SEM and TEM analysis. The eggs, embryos and juveniles are attached in close vicinity of each parapodial complex in a position immediately below each ventral cirrus and are connected to the female by a cup like structure. The formation of this adhesive disk is linked to secretory cells scattered between dermal cells of ripe female. This adhesive disk is present only in sexually mature animals and is considered as epitokous structure. The evolutive significance of ventral and dorsal attachment found within the Exogoninae is also discussed. © 2003 Elsevier Ltd. All rights reserved

Some Like It Fat: Comparative Ultrastructure of the Embryo in Two Demosponges of the Genus Mycale (Order Poecilosclerida) from Antarctica and the Caribbean

0000-0002-7993-1523© 2015 Riesgo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License [4.0], which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The attached file is the published version of the article

The Whereabouts of an Ancient Wanderer: Global Phylogeography of the Solitary Ascidian Styela plicata

Genetic tools have greatly aided in tracing the sources and colonization history of introduced species. However, recurrent introductions and repeated shuffling of populations may have blurred some of the genetic signals left by ancient introductions. Styela plicata is a solitary ascidian distributed worldwide. Although its origin remains unclear, this species is believed to have spread worldwide by travelling on ship's hulls. The goals of this study were to infer the genetic structure and global phylogeography of S. plicata and to look for present-day and historical genetic patterns. Two genetic markers were used: a fragment of the mitochondrial gene Cytochrome Oxidase subunit I (COI) and a fragment of the nuclear gene Adenine Nucleotide Transporter/ADP-ATP Translocase (ANT). A total of 368 individuals for COI and 315 for ANT were sequenced from 17 locations worldwide. The levels of gene diversity were moderate for COI to high for ANT. The Mediterranean populations showed the least diversity and allelic richness for both markers, while the Indian, Atlantic and Pacific Oceans had the highest gene and nucleotide diversities. Network and phylogenetic analyses with COI and ANT revealed two groups of alleles separated by 15 and 4 mutational steps, respectively. The existence of different lineages suggested an ancient population split. However, the geographic distributions of these groups did not show any consistent pattern, indicating different phylogeographic histories for each gene. Genetic divergence was significant for many population-pairs irrespective of the geographic distance among them. Stochastic introduction events are reflected in the uneven distribution of COI and ANT allele frequencies and groups among many populations. Our results confirmed that S. plicata has been present in all studied oceans for a long time, and that recurrent colonization events and occasional shuffling among populations have determined the actual genetic structure of this species

Choanocyte chambers in unreleased buds of Tethya seychellensis. (Wright) (Porifera, Demospongiae)

The buds of the sponge Tethya seychellensis, collected in May 2006 from South Male Atoll (Maldives), have been processed for histological and ultrastructural investigations. Buds show three stages, different in morphology and size, reflecting different steps of their differentiation: round-shaped buds (stage I), cylindrical-shaped buds (II) and thin-stalked spear-shaped buds (III). Cells with inclusions, belonging to the category of spherulous cells, are the main components of these buds. At stage I, some irregularly shaped cells, probably precursors of the choanocytes, are able to engulf and remove portions of the collagen matrix, a feature which could be interpreted as an initial step in choanocyte differentiation. Tethya seychellensis stands out among the other congeneric species owing to the occurrence of choanocyte chambers in the unreleased buds, as observed at stages II and III. The precocious differentiation of the choanocyte chambers allows the detached bud to act as a young functional sponge, thus enhancing the survival chances of these filter-feeding organisms. The structure of the buds of T. seychellensis stresses once more that in the genus Tethya the organisation of the buds represents a species-specific process, which varies according to the plasticity and the morphogenetic potentiality expressed by a single species