Chimera chicken gives insight into avian sex cell development

Birds of a feather...: This avian gynandromorph exhibits hen traits on its right side, and rooster traits on its left side, and is giving scientists insight into sex development in birds and other animals.
Birds of a feather...: This avian gynandromorph exhibits hen traits on its right side, and rooster traits on its left side, and is giving scientists insight into sex development in birds and other animals.Courtesy Roslin Institute, University of Edinburgh
I must not have been paying attention lately because it appears there are some real bizarre chickens out there that are half rooster and half hen. I don't mean a jumbled mix where a bird exhibits some hen characteristics here and a couple rooster traits there – no, these are barnyard oddballs where one side is a female, and the other side is a male. It’s as if someone sliced a bird of each sex smack dab down the middle and sewed the two different halves together. A real, feathered chimerical freak show. These natural anomalies (known as gynandromorphs) appear in other life forms and have apparently been around for centuries.

Anyway, new research out of the University of Edinburgh’s Roslin Institute indicates that differences in male and female chicken cells - rather than hormones triggering genes – play a major role in determining sexual development in birds.

“This research has completely overturned what we previously thought about how sexual characteristics were determined in birds. We now believe that the major factors determining sexual development are built into male and female cells and derive from basic differences in how sex chromosome genes are expressed. Our study opens a new avenue for our understanding of sexual development in birds.” - Dr Michael Clinton

With mammalian development – including humans – after the two gamete cells (sperm and ova) fuse, the zygote’s somatic cells begin to grow and develop into whatever body plan it’s forming. The embryo remains sexually indifferent until certain genes switch on and sex-determining hormones are secreted. And for a long time it was thought all vertebrates developed in this same manner. But it looks like birds follow a completely different plan of action. Avian somatic cells don’t wait to get their cue from the gonads, and seem to already “know” their own sexual identity before any sex-determining hormones are secreted.

Named cell autonomous sex identity (CASI), the phenomenon runs counter to the prevailing ideas of development of sexual traits in birds, and explains why a gynandromorph can display female traits on one side and rooster traits on the other in the same bird. The male or female cells are dominant on the respective sides.

“It also means we must now reassess how this developmental process occurs in other organisms. There is already some evidence that organs such as the heart and brain are intrinsically different in males and females and birds may provide a model for understanding the molecular basis for these gender differences.” – Dr. Michael Clinton

The findings which appeared in the journal Nature, could also lead to answering why one gender is predisposed to certain diseases while the other isn’t. But whether they’ll be able to explain why gynandromorph chickens make the best fighting birds in cockfights is another question.

Roslin Institute release
More about gynandromorphs
More about gonads story

Your Comments, Thoughts, Questions, Ideas

Max's picture
Max says:

Much more noise than real scientific discovery. Practically all the key points of the paper were well known earlier. The only strongest side of the work is that the guys used the modern methods to study the phenomenon of gynandromorphism, and I am afraid that if the main conclusion of the work about autonomy of the sex determination process in birds turns out wrong such kind of "discoveries" will slow rather than promote to the search of the real mechanisms involved. It seems to me that the main claim of the work is not grounded sufficiently. Not to be unfounded I will try to ground my point of view only in regard of the main calim:

1. Notation from the paper: “The fact that female chicken cells in an environment and location that induces testicular development cannot be recruited into the functionally ‘male’ Sertoli cell compartment, and male cells in an ovary-inducing environment are excluded from a functionally ‘female’ compartment, strongly supports the suggestion that chicken somatic cells possess a cell-autonomous sexual identity.”

As far as known only stem cells upon transplantation into the host organism can be reprogrammed to function in accordance with the environment and location of the graft, but not differentiated somatic cells.

2. Notation from the paper: “Our findings are in contrast with those from mammalian mixed-sex chimaeras, where XX cells can become functional Sertoli cells and XY cells can become functional granulosa cells.”

In the mentioned articles the chimeras were produced at the very early stages of development, when the cells are totipotent: injection chimeras in the first work were obtained by introducing male embryonic stem cells into unsexed host blastocysts and aggregation chimeras in the second work also were produced by aggregating of cells from blastocyst stage. It is unreasonable to compare the results obtained from primary mice chimeras (in the mentioned works) and secondary chicken chimeras (in the discussed work), when the differentiated somatic tissues were grafted. “…mouse chimaeras, derived by introducing male embryonal stem cells into unsexed host blastocysts, were examined to determine whether gonadal sex was correlated with the sex chromosome composition of particular cell lineages. Both XX and XY cells were found in all gonadal somatic tissues but Sertoli cells were predominantly XY and granulosa cells predominantly XX. Male chimaeras with XX↔XY testes were either sterile or less fertile than chimaeras with testes composed entirely of XY cells. This impaired fertility was associated with the loss of XY germ cells in atrophic seminiferous tubules. Since this progressive lesion was correlated with a high proportion of XX Leydig cells, we suggest that XX Leydig cells are functionally defective, and unable to support spermatogenesis.” (Patek, C. E. et al.1991.). As we can make sure from the previous annotation even when the mouse chimeras were produced from the embryo stem cells the resulting XX Leydig cells were functionally defective, and unable to support spermatogenesis.
“Importantly, GPI-1A follicle cells were identified in more than half the follicles from an XX↔XY female in which the GPI-1A component was XY, supporting an earlier conclusion of Ford et al. (1974) that XY cells can contribute to the follicles of XX↔XY female mice.” (Burgoyne, P. S., Buehr, M. & McLaren,1988).

The presented in the annotation results are in full concordance with the results of numerous works on production of secondary chimeras (early works till 1990 when Petitte et al. first produced viable germ line chimeras (Petitte J. N., et al. 1990). and blastodermal chimeras in chickens. Haffen (1975) investigated the differentiation of germ cells in the gonads of the opposite sex in the chick by a combination of chimera production and organ culture experiments. He observed male germ cells in the ovary and female germ cells in the testes.
Kagami et al. (1995) produced chimeric chickens utilizing blastodermal cells in which the donor cells were derived from individual embryos and the genetic sexes of donor and recipient were identified by in situ hybridization using a W-chromosome-specific DNA probe. Injection of female blastodermal cells into male recipients produced only male chimeras and donor derived offspring were obtained from the ZZ (ZW) chimeras. This was the first experiment that demonstrated that the female germ cell could differentiate into functional gametes in the male gonad. Tajima et al., 1993; Naito et al., 1994a; 1994b; 1998, 1999 produced same sex and mixed sex chimeras by transfer of PGCs isolated from embryonic blood and also obtained donor derived offspring. However, the frequency of germline chimerism and the rate of germline transmission in mixed sex chimeras were lower than in the same sex chimeras. These results indicated the differentiation of female (ZW) PGCs in male (ZZ) gonads and male PGCs in female gonads were partially restricted.

posted on Thu, 11/10/2011 - 8:36pm

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