Expression analysis of steroid hormone receptor mRNAs during zebrafish embryogenesis

We have analyzed by qRT-PCR and/or RT-PCR the abundance and degradation rate of maternal mRNAs for nine steroid hormone receptors and their possible replacement by corresponding embryonic transcripts in both ovulated oocytes and embryos of zebrafish collected at 0, 1, 2, 4, 8, 12, 24 and 48 h post-fertilization (hpf). The mRNAs encoded the nuclear receptors for progesterone (pr), androgen (ar), estrogen (er alpha, er beta 1 and er beta 2), glucocorticoids (gr), mineralocorticoids (mr) and the membrane progestin receptor-alpha and beta (mpr alpha and beta). gr mRNA was the most abundant maternal transcript in oocytes and early embryos followed by er beta 2 and ar mRNAs. They declined during the first 8 hpf, being replaced, thereafter, by the embryonic messengers. er beta 1 and mr transcript levels were low until 8 hpf, but increased steadily during embryonic transcription from 24 to 48 hpf. pr transcripts were detectable only in ovulated oocytes and at 24 and 48 hpf. At these stages, there was a slight increase of er alpha mRNA that initially was very low. mPr alpha and beta mRNAs were expressed in ovulated oocytes and faintly persisted during the first 4 hpf. There was no subsequent embryonic expression of these transcripts. The possible involvement of maternal mRNAs for glucocorticoid and sex hormone receptors in the programming of early zebrafish development is intriguing, since they mainly occur at stages in which gene replication predominates over transcription.