Cytogenetic studies on diploid and triploid oogenesis in interspecific hybrid fish between <i>Oryzias latipes</i> and <i>O. curvinotus</i>

Abstract

Abstract Interspecific hybrid females produced from reciprocal crosses between Oryzias latipes and O. curvinotus show reduced fertility, but they do lay eggs. When fertilized by sperm from one of the parental species, such eggs developed into triploid males and females. To confirm the ploidy of the eggs spawned by the hybrid females we examined the karyotypes of backcross and gynogenetic offspring. The karyotype of the China‐West Korean medaka, O. latipes sinensis , was 2n, with 46 chromosomes, including a pair of “large” metacentrics, whereas O. curvinotus had 2n, 48, without such a chromosome pair. F 1 hybrids had a karyotype 2n, 47, with one large chromosome. When the hybrid females were backcrossed to O. l. sinensis , the offspring had a karyotype of 3n, 70, with two large chromosomes. By contrast, when backcrossed to O. curvinotus, the progeny of the hybrid females had a karyotype of 3n, 71, with one large chromosome. After gynogenetic treatment with UV‐irradiated sperm of rainbow trout, the eggs spawned by the hybrid females developed into diploid (2n, 47) fish with one large metacentric chromosome. These results indicate that the hybrid females spawn unreduced diploid eggs with one complete chromosome set for both parental species. A triploid female, in contrast to the diploid hybrid females, spawned eggs with reduced chromosome number, giving rise to hyperdiploid embryos when fertilized by sperm. Transplantation of scales and electrophoretic analysis of proteins indicated that a hybrid female and her gynogenetic offspring can be regarded as “clones” with identical sets of chromosomes. © 1995 Wiley‐Liss, Inc.