Sex chromosomes in the spiny eel (Mastacembelus aculeatus) revealed by mitotic and meiotic analysis.
Lower vertebrates like fish exhibit tremendous diversity in sex determination. There are wide interplays between environment-dependent sex differentiation ranging from natural hermaphroditism to sex reversal and genetic sex determination. Diverse systems of male and female heterogamety coexist in fish and sex chromosomes are rarely distinguishable in morphology. Here we show that the spiny eel ((Mastacembelus aculeatus) of the Perciformes, has evolved highly heteromorphic X and Y chromosomes. The metacentric X and Y chromosomes are the largest among 24 homologous pairs, differ from each other in size and morphology, and become distinct after C-banding because of conspicuous heterochromatin blocks which exhibit alternate distribution around the centromeric region. Chromosome painting using probes from the microdissected X chromosome revealed sequence homology between X and Y. During the pachytene stage of meiosis the X and Y form a bivalent. However, their synapsis is delayed which is particularly evident in one terminus. Therefore, the X and Y have resulted from a pericentric inversion in the Y. We conclude that M. aculeatus represents an example of a highly advanced stage of sex chromosome evolution in fish.
Liu
J D
JD
School of Life Sciences, Wuhan University, Wuhan 430072 Hubei, P.R. China.
Yi
M S
MS
Zhao
G
G
Zhou
F
F
Wang
D Q
DQ
Yu
Q X
QX
D000818:Animals; D005075:Biological Evolution; D015139:Blotting, Southern; D004524:Eels / Q000235:genetics*; D005260:Female; D017404:In Situ Hybridization, Fluorescence; D008297:Male; D008540:Meiosis; D008938:Mitosis; D016133:Polymerase Chain Reaction / Q000379:methods; D019849:Sex Determination Processes; D014960:X Chromosome; D014998:Y Chromosome