Metabolic cooperation following fusion of starfish ootid and primary oocyte restores meiotic-phase-promoting activity

A rare case of intra-ovarian oocyte maturation

The intra-ovarian presence of ootids, i.e. female gametes that have completed meiosis, is considered exceptional in the animal kingdom. The present study explores the first such case to be reported in a sea cucumber (Echinodermata: Holothuroidea). In the overwhelming majority of animals, including holothuroids, oocytes (i.e. immature female gametes) that are developing in the ovary undergo a primary arrest at the prophase stage of meiosis, which may last from days to decades. In free-spawning taxa, this arrest is normally lifted only during or shortly before transit in the gonoduct, when gamete release (spawning) is imminent. However, oocytes of the holothuroid Chiridota laevis were discovered to have resumed the second meiotic division including the completion of germinal vesicle breakdown and polar-body expulsion inside the ovary, effectively reaching the ootid stage concomitantly with ovulation (i.e. escape from follicle cells) prior to spawning. The potential drivers and significance of this exceptionally rare case of full intra-ovarian oogenic maturation are discussed.

Intracellular Pathways of Holothuroid Oocyte Maturation Induced by the Thioredoxin Trx-REES

In holothuroids, oocyte maturation is stopped in ovaries at the prophase I stage of meiosis. In natural conditions, the blockage is removed during the spawning by an unknown mechanism. When oocytes are isolated by dissection, the meiotic release can be successfully induced by a natural inducer, the REES (i.e., Rough Extract of Echinoid Spawn) that is used in aquaculture to obtain viable larvae in mass. A thioredoxin has recently been identified in the REES as the molecule responsible for holothuroid oocyte maturation. As a redox-active protein, thioredoxin is thought to reduce target proteins within the oocyte membrane and initiate an intracellular reaction cascade that leads to the unblocking of the oocyte meiosis. Our results allow us to understand additional steps in the intracellular reaction cascade induced by the action of thioredoxin on oocytes. Pharmacological agents known to have activating or inhibiting actions on oocyte maturation have been used (Forskolin, Isobutylmethylxanthine, Hypoxanthine, 6-dimethyaminopurine, Lavendustin, Genistein, Roscovitine, Cycloheximide). The effects of these agents were analysed on oocytes of the holothuroid Holothuria tubulosa incubated with or without REES and were compared to those obtained with another reducing agent, the dithiothreitol. Our results demonstrated that, at the opposite of dithiothreitol-induced oocyte maturation, thioredoxin-induced oocyte maturation is cAMP independent, but dependent of the presence of calcium in the seawater. Both pathways of induction require the activation of protein serine/threonine kinases.

SPLICE SITE SELECTION IN PLANT PRE-mRNA SPLICING

The purpose of this review is to highlight the unique and common features of splice site selection in plants compared with the better understood yeast and vertebrate systems. A key question in plant splicing is the role of AU sequences and how and at what stage they are involved in spliceosome assembly. Clearly, intronic U- or AU-rich and exonic GC- and AG-rich elements can influence splice site selection and splicing efficiency and are likely to bind proteins. It is becoming clear that splicing of a particular intron depends on a fine balance in the "strength" of the multiple intron signals involved in splice site selection. Individual introns contain varying strengths of signals and what is critical to splicing of one intron may be of less importance to the splicing of another. Thus, small changes to signals may severely disrupt splicing or have little or no effect depending on the overall sequence context of a specific intron/exon organization.

The oocyte metaphase arrest

Usually, oocyte meiosis reinitiation appears as a two step process during which release from the prophase block is followed by a second arrest in metaphase I or II. In this review, we will examine the mechanisms required to maintain the metaphase arrest and stabilize MPF activity at this stage. Then, we will analyse the processes required to exit from the metaphase block. These may drive the cells forward to the metaphase-anaphase transition, as a result of fertilization, activation or protein synthesis inhibition. Instead, inhibiting protein phosphorylation drives the oocyte back to interphase. All these treatments result in derepression of DNA synthesis.

Electric fusion of unfertilized starfish oocytes

An electric power device to fuse starfish oocytes was constructed. The outputs were supplied to a pair of parallel platinum wires in a solution of mannitol to which small amounts of salts were added. Oocytes of the starfish Asterina pectinifera, deprived of the vitelline envelope, were fused by several repetitions of a combination of 2.5 MHz AC field for a few seconds and a 50 micros DC pulse. Observation of fusing pairs of immature oocytes revealed that: (i) the oocyte placed near to the cathode forms a bulge at the surface facing the anode when subjected to DC pulses and, with subsequent DC pulses, a similar bulge is formed on another oocyte; and (ii) the pair of bulges then fuse together leading to fusion of the main body of the two oocytes. The conjugate of maturing oocytes soon became a single sphere, usually within 10 min, but this process toward spherical form paused when the oocyte was extruding its polar bodies. The conjugate of immature oocytes took 1 h to become a single sphere. The fusion did not disturb the progress of meiotic events, but electric pulses at an intensity suitable for the fusion often activated maturing oocytes and mature eggs.