Maturation and ovulation of Japanese quail oocytes under in vitro conditions

New insights into implication of the SLIT/ROBO pathway in the prehierarchical follicle development of hen ovary

The SLIT/Roundabout (ROBO) pathway is involved in follicle development of mammalian ovary, and 2 secreted hormones activin A and inhibin A have potential roles in modulation of the SLIT/ROBO system, but the related actions remain poorly understood in bird. The aims of the present study were to examine the spatial and temporal expression of the SLIT ligand genes (SLIT1, SLIT2, and SLIT3) and their receptor ROBO1, ROBO2, ROBO3, and ROBO4 genes in various-sized prehierarchical follicles during hen ovary development and the effects of activin A and inhibin A on the expression of these genes in the cultured hen follicles. Our result demonstrated that the transcripts of the 3 SLIT genes were highly expressed in the developing follicles and expression patterns of the SLIT transcripts were different from those of ROBO genes detected by real-time quantitative reverse transcriptase PCR. Both SLIT and ROBO transcripts were predominantly expressed in oocytes and granulosa cells from the prehierarchichal follicles examined by in situ hybridization. The localization for SLIT and ROBO proteins was revealed by immunohistochemistry similar to the spatial distribution of their transcript. In cultured follicles (4 to 8 mm in diameter), the expression levels of SLIT and ROBO members are hormonally regulated by activin A (10 ng/mL) and/or inhibin A (20 ng/mL) after treatment for 24 h. However, the expression of only SLIT2, SLIT3, and ROBO3 mRNA presented a directly opposite response to activin A and inhibin A hormones. These results indicate that SLIT/ROBO pathway is implicated in the prehierarchical follicular development of the hen ovary by an intrafollicular autocrine and/or paracrine action, and is influenced by activin A and inhibin A hormones.

Serotonin receptors are selectively expressed in the avian germ cells and early embryos

The expression of nine serotonin (5-HT) receptor transcripts was studied using reverse transcription polymerase chain reaction (RT-PCR) in germ cells, cleavage and gastrulation stages of Japanese quail, and qPCR for 5-HT3 and 5-HT4 receptors in oocytes and embryos. We show the presence/absence of nine serotonin transcripts known in birds for receptors 5-HT1A, 5-HT1F, 5-HT2B, 5-HT2C, 5-HT3, 5-HT4, 5-HT5A, 5-HT6 and 5-HT7A in avian germ cells and early embryos. The absence of 5-HT3 and 5-HT5A in primordial germ cells and of 5-HT3 and 5-HT7A in sperm is characteristic. All transcripts appeared in oocytes at all stages (except for 5-HT3 and 5-HT5A transcripts) and all were present in cleaving embryos and at gastrulation, except for 5-HT3, which was permanently observed as late as in stage 4. Interestingly, 5-HT3 and 5-HT5A receptors accumulated in 3-mm and F1 oocytes but were degraded at ovulation and started to be re-transcribed in cleavage stage II embryos and beyond. The selective appearance of 5-HT receptors in germ cells and early embryos supports the hypothesis that serotonin may act as a signalling molecule at early stages of germ line and embryo differentiation via individual receptors present during different stages, when specialized communication systems are not yet developed.

Review. Meiotic drive and sex determination: molecular and cytological mechanisms of sex ratio adjustment in birds

Differences in relative fitness of male and female offspring across ecological and social environments should favour the evolution of sex-determining mechanisms that enable adjustment of brood sex ratio to the context of breeding. Despite the expectation that genetic sex determination should not produce consistent bias in primary sex ratios, extensive and adaptive modifications of offspring sex ratio in relation to social and physiological conditions during reproduction are often documented. Such discordance emphasizes the need for empirical investigation of the proximate mechanisms for modifying primary sex ratios, and suggests epigenetic effects on sex-determining mechanisms as the most likely candidates. Birds, in particular, are thought to have an unusually direct opportunity to modify offspring sex ratio because avian females are heterogametic and because the sex-determining division in avian meiosis occurs prior to ovulation and fertilization. However, despite evidence of strong epigenetic effects on sex determination in pre-ovulatory avian oocytes, the mechanisms behind such effects remain elusive. Our review of molecular and cytological mechanisms of avian meiosis uncovers a multitude of potential targets for selection on biased segregation of sex chromosomes, which may reflect the diversity of mechanisms and levels on which such selection operates in birds. Our findings indicate that pronounced differences between sex chromosomes in size, shape, size of protein bodies, alignment at the meiotic plate, microtubule attachment and epigenetic markings should commonly produce biased segregation of sex chromosomes as the default state, with secondary evolution of compensatory mechanisms necessary to maintain unbiased meiosis. We suggest that it is the epigenetic effects that modify such compensatory mechanisms that enable context-dependent and precise adjustment of primary sex ratio in birds. Furthermore, we highlight the features of avian meiosis that can be influenced by maternal hormones in response to environmental stimuli and may account for the precise and adaptive patterns of offspring sex ratio adjustment observed in some species.

Isolation of chicken homolog of theFOXL2gene and comparison of its expression patterns with those of aromatase during ovarian development

Mutations in the forkhead transcription factor gene FOXL2 are involved in ovarian failure, which occurs in human BPES syndrome. This syndrome presents a sexually dimorphic expression, specific to the ovary in several vertebrates. We cloned the open reading frame of chicken FOXL2 (cFoxL2) and studied cFoxL2 expression in developing gonads and during adulthood to examine the role of FOXL2 in ovarian differentiation and function in birds. The spatial and temporal dynamics of cFoxL2 and aromatase expression were analyzed in parallel by using real-time quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry in attempt to investigate the possible role of cFoxL2 in the regulation of aromatase. The expression patterns of cFoxL2 and aromatase transcripts were highly correlated during the sex-differentiation period (4.7-12.7 days of incubation). Aromatase and cFoxL2 proteins were colocalized in the medullar part of female gonads on embryonic day 14. Fourteen days after hatching, cFoxL2 protein was mainly detected in granulosa cells of developing follicles. In adult ovary follicular envelopes, apart from granulosa cells, cFoxL2 transcript and protein were detected at lower levels in theca cells where aromatase was present. A high level of cFoxL2 transcription was also observed in maturing and ovulated oocytes. Our results confirm that FoxL2 is an early regulator of ovarian development in birds and may be involved in aromatase transcription regulation.

Presence and developmental regulation of serotonin N-acetyltransferase transcripts in oocytes and early quail embryos (Coturnix coturnix japonica)

By RT-PCR two transcripts for arylalkylamine N-acetyltransferase (AA-NAT; serotonin N-acetyltransferase; EC 2.3.1.87), the key enzyme in melatonin synthesis, were found, for the first time, in the oocytes and blastoderms from freshly laid eggs (323- and 238-bp RT-PCR products), and one (238-bp product) in the pineal gland of Japanese quail (Coturnix coturnix japonica). The two products differed by an intron of 85-bp present in the 323-bp band and absent from the 238-bp band. The identity of the products was confirmed by restriction analysis and sequencing. The ratio of the 323:238-bp bands changed during oogenesis from approximately 17:1 in small 3-mm oocytes to approximately 4:1 in immature vitellogenic oocytes and approximately 1:1 in mature, preovulatory oocytes; it was reversed to approximately 0.2:1 in blastoderms from fertile freshly laid eggs, corresponding to embryo of approximately 40,000 cells. It is proposed that the longer 323-bp product, containing an intron, represents a translationally inactive form of the transcript, stored in maternal RNA. The shorter 238-bp product lacking an intron may represent the mature active AA-NAT mRNA found in the pineal gland and in early embryos, and-to a lower proportion-in older oocytes. These data constitute the first direct proof of an intron sequence in maternal RNA of avian oocyte. It is possible that differential processing of the immature mRNA is part of a transcriptional regulation mechanism of AA-NAT activity. A possible role of extrapineal melatonin in early avian development is discussed.

Development of embryos from in vitro ovulated and fertilized oocytes of the quail (Coturnix coturnix japonica)

The development of quail embryos obtained after in vitro fertilization of oocytes ovulated in vitro was investigated. About 40% of the specimens, after 18-20 hr of incubation, had undergone cleavage to reach stages IV-VI when viewed under a stereo microscope. However, only 36% of these embryos contained normal, DAPI-stained nuclei when observed under a fluorescent microscope; the other 64% showing a morphologically normal cleavage pattern did not contain nuclei. Control unfertilized oocytes, ovulated in vitro and cultured for the same time, also sometimes attained the morphologically correct stages IV-VI but their "blastomeres" were always devoid of nuclei. Therefore, it is advisable to monitor early avian embryos for the presence of nuclei when assessing development in culture. The results demonstrate, for the first time, that cytoplasmic segmentation can occur in the absence of nuclear divisions in the germinal disc of the quail and show the existence and significance of ooplasmic maternal information in birds. This phenomenon is also known for sea urchin and frogs. It is indicative of the role of maternal information in early development. The in vitro method presented here links the steps of ovulation and fertilization with the early cleavage stages under in vitro conditions and may be useful in studying mechanisms of fertilization and differentiation in birds as well as in obtaining transgenic birds by DNA injection or application of foreign, DNA-carrying sperm.