Lampbrush chromosomes and chiasmata of sex-reversed crested newts

Abstract

Triturus cristatus carnifex provides a particularly clear example of sexual dimorphism for chiasma frequency and localisation. Oocytes from normal XX females routinely carry one proximal chiasma on each arm of their lampbrush bivalents. Spermatocytes from normal XY males have more numerous and relatively distal chiasmata. Lampbrush chromosomes from the oocytes of sex-reversed XY neofemales are found to resemble those from normal oocytes in having one proximal chiasma on each bivalent arm. A comparison of particular markers on the heteromorphic long arm of chromosome 1 provides evidence to equate the lampbrush 1A to somatic 1A, and confirms previous reports that lampbrush chromosome 1A is slightly longer than 1B. The XY sex bivalent of neofemales does not show any obvious heteromorphy of recognised marker loops.

Amphibian sex determination and sex reversal

“…We have confirmed the feminizing effect of estradiol on T. c. carnifex larvae and taken advantage of the identifiable Y chromosome to isolate XY females, in order to characterize their lampbrush chromosomes [44]. The mildest effective treatment found was exposure to 25 mg/l of estradiol for 8 -9 weeks, from early feeding larvae with three fingers to beyond the stage when all five toes were present.…”

“…Neither expectation could be verified, as 24 females and 15 males were scored at metamorphosis and no YY specimen was found among 26 tailtip biopsies, leading to the conclusion that some form of meiotic drive prevents transmission of the Y chromosome into mature eggs. That implies nonrandom segregation of the Y chromosome into a polar body, since the oocytes from this neofemale carried a normal complement of lampbrush chromosomes [44].…”

Long-term sex reversal by oestradiol in amniotes with heteromorphic sex chromosomes

“…Our findings instead point to a difference between the mechanisms controlling sex determination in birds and mammals versus other vertebrates: oestradiol apparently cannot override male sex determination in birds and mammals, but can in fish, amphibians and reptiles (Wallace et al 1997;Devlin & Nagahama 2002; this study). Mammals have evolved regulatory genes that control the initiation of sex determination (i.e.…”

Synaptonemal complex karyotype of zebrafish

“…We are intrigued by the possibility that the last round of genomic doubling in zebrafish evolution, as documented by Postlethwaite et al (1999), might have disrupted an ancestral genetic sex determination mechanism leaving relics of sex chromosomes. The features of chromosome 3 mentioned above would make it a candidate relict sex chromosome, like chromosome 1 of crested newts (Wallace et al, 1997).…”