Production of Diploid Eggs through Premeiotic Endomitosis in the Hybrid Medaka between Oryzias latipes and O. curvinotus

Abstract

ABSTRACT-A hybrid medaka between Oryzias latipes and O. curvinotus spawns diploid eggs. We examined the cytological mechanisms of diploid egg formation in this hybrid. Oocytes in the hybrid passed through the first and second meiotic divisions and excluded two polar bodies, associated with reduction of the DNA content in oocytes. Each germinal vesicle in vitellogenic oocytes of the hybrid had 48 chromosomes with bivalent chiasmata, precisely twice the number of chromosomes in normal oocytes. These results suggest that before meiosis the chromosomes are doubled by mitosis without cytokinesis, that is, endomitosis, and that the resulting tetraploid oogonia undergo normal meiosis to produce diploid eggs. Except for a few vitellogenic oocytes that are probably derived from endomitotic oogonia, most oocytes were arrested at the zygotene stage in the hybrid ovary, suggesting the existence of checkpoint control that ensures the pairing of homologous chromosomes at prophase I, a situation differing from that in the male in which the checkpoint is at metaphase I. A sac-like structure was characteristic of the hybrid ovary. Although this structure was observed only in the hybrid, it may be a native component of the medaka ovary but difficult to observe because of its deformed outer structure due to enlargement of the inside oocytes. The present study confirms that this hybrid medaka provides a useful experimental system for investigations into the mechanisms of oogenesis and basic architecture of the ovary, which are difficult to analyze by using normal medaka.

A Cryptic Clonal Line of the Loach Misgurnus anguillicaudatus (Teleostei: Cobitidae) Evidenced by Induced Gynogenesis, Interspecific Hybridization, Microsatellite Genotyping and Multilocus DNA Fingerprinting

“…In the loach, unreduced triploid eggs are formed in triploid (diploid female x tetraploid male) individuals by this system (Zhang et al, 1998). A similar mechanism has been found in triploid Poeciliopsis (Cimino, 1972) and artificial medaka hybrids between Oryzias latipes and O. curvinotus (Shimizu et al, 2000). In this system synapses and recombination occur essentially between the sister replicate chromosomes produced by the endomitosis.…”

“…The occurrence of unisexual, clonal and/ or polyploid biotypes caused by apparent hybridization has been reported in fish of the genera Phoxinus (Goddard and Dawley, 1990), Fundulus (Dawley, 1992), Rutilus (Alves et al, 1998(Alves et al, , 1999, Cobitis (Kim and Lee, 2000) and Oryzias (Shimizu et al, 2000). Since the unisexual reproduction is generally a rare phenomenon in vertebrates, it is difficult to identify it in wild populations without careful surveillance of unusual occurrences such as aberrant sex ratios or the apparent appearance of hybrids and/or polyploids.…”

Disruption of normal meiosis in artificial inter-populational hybrid females of Misgurnus loach

“…But they may act as a supplier of unreduced diploid eggs, which give rise to triploid development after fertilization with sperm of bisexual diploids in nature. The production of such unreduced eggs by interspecific hybridization has been reported in teleosts including Salmo species (Johnson and Wright, 1986), Phoxinus species (Dawley et al, 1987;Dawley and Goddard, 1988;Goddard and Dawley, 1990), sunfish (Dawley et al, 1985;Dawley, 1987), medaka species (Sakaizumi et al, 1993;Kurita et al, 1995;Shimizu et al, 2000), and Fundulus species (Dawley, 1992). Similar results were also reported in Cnemidophorus in Reptiles (Dessauer and Cole, 1989;Moritz et al, 1989).…”

GLUCAN SYNTHASE-LIKE8 and STEROL METHYLTRANSFERASE2 Are Required for Ploidy Consistency of the Sexual Reproduction System inArabidopsis

“…For example, in oogonia of natural loach clones and hybrid fish species, the ectopic induction of premeiotic endomitosis leads to 2n eggs (Shimizu et al, 2000;Liu et al, 2004;Itono et al, 2006). In all these cases, however, the underlying cytological defect is unknown and may not relate to defects in cytokinesis.…”