Mesostoma ehrenbergii spermatocytes – A unique and advantageous cell for studying meiosis

Abstract

Mesostoma ehrenbergii have a unique male meiosis: their spermatocytes have three large bivalents that oscillate for 1-2 h before entering into anaphase without having formed a metaphase plate, have a precocious ('pre-anaphase') cleavage furrow, and have four univalents that segregate between spindle poles without physical interaction between them, that is via 'distance segregation'. These unique and unconventional features make Mesostoma spermatocytes an ideal organism for studying the force produced by the spindle to move chromosomes, and to study cleavage furrow control and 'distance segregation'. We review the literature on meiosis in Mesostoma spermatocytes and describe our current research with Mesostoma spermatocytes, rearing the animals in the laboratory using methods that described in our companion article [Hoang et al. (2013); Cell Biol Int].

Evidence for Karyotype Polymorphism in the Free-Living Flatworm, Macrostomum lignano, a Model Organism for Evolutionary and Developmental Biology

“…etrusca , revealed aneuploidy and polyploidy transmitted through the female germ line, whereas worms produced only normal spermatozoa [ 56 ]. Studies of another free-living flatworm, Mesostoma ehrenbergi , revealed particular features of meiosis (extensive chromosome oscillations, the absence of a metaphase plate, distance segregation of univalents, and a precocious ‘pre-anaphase’ cleavage furrow [ 57 , 58 ]), which potentially can lead to formation of aneuploid gametes. While the features of meiosis in M .…”

Chromosome interaction over a distance in meiosis

“…The best-studied example of distance segregation of apparent autosomes (i.e. partners appear to be the same size, and there is no visible difference between chromosome complement in oocytes and spermatocytes) comes from the flatworm Mesostoma ehrenbergii [ 30 ]. In meiosis I of these flatworms, three homologous pairs of chromosomes form bivalents (chromosomes 1, 3 and 4), and two homologous pairs do not form bivalents, but remain univalent (chromosomes 2 and 5; figure 5 ).…”

“…There may be both coordinated positioning of partner chromosomes, and coordinated positioning of non-partner chromosomes [ 11 , 34 ]. Additionally, univalents and half-bivalents prior to anaphase I will move away from and then back to the same pole, apparently detaching from the pole with which they were associated and then reattaching to that same pole [ 11 , 30 ]. Anaphase movements ( figure 6 c ) start in the middle of oscillations, with no apparent stable metaphase prior to anaphase I [ 30 , 34 ].…”

“…Additionally, univalents and half-bivalents prior to anaphase I will move away from and then back to the same pole, apparently detaching from the pole with which they were associated and then reattaching to that same pole [ 11 , 30 ]. Anaphase movements ( figure 6 c ) start in the middle of oscillations, with no apparent stable metaphase prior to anaphase I [ 30 , 34 ]. The trigger that leads to onset of anaphase I is unknown, but it seems to be associated with a euploid distribution of chromosomes.…”

Evidence of Non-microtubule Spindle Forces in Mesostoma ehrenbergii Spermatocytes

“…The chromosomes never form a metaphase plate: oscillations continue until anaphase. At anaphase the bivalent oscillations end abruptly and the segregating chromosomes move toward the two poles at speeds of approximately 1 μm/min ( Fuge, 1987 , 1989 ; Ferraro-Gideon et al, 2013 , 2014 ). For clarity, we must describe several other unusual behaviors in these cells.…”