Subject: orioles/genetics Date: May 04 08:00:00 1995 From: "Michelsen, Teresa" - TEMI461 at ecy.wa.gov
I have to respond to something Burt said yesterday "we all have X and Y chromosomes". We DON"T all have both chromosomes (unless by WE you mean only males :-)). Male humans have XY and females have XX. In birds, as you pointed out, males have WW and females have WZ. This single fact accounts for ALL sex-related differences between males and females, either directly or indirectly. If you think about it, no gene that is not on a sex-chromosome can account for a M/F difference ALL BY ITSELF, since genes on these chromosomes are passed on randomly to offspring, without regard to sex. However, genes on these other chromosomes can be expressed differently between M and F, as you stated. All this really means is that the trait can only appear in the presence of some other enzyme or hormone. In the case of a male/female trait, this is generally a sex hormone, whose production is in turn regulated by other genes on the sex chromosomes. I looked up some interesting traits (in my Van Nostrand scientific encyclopaedia; my textbooks are all at work) and thought through some patterns as they apply to birds, so those that are interested in plumage characteristics as determined by genetics, read on - others may find it too long.
Essentially, there are three types of sex-related differences in humans: Sex-Linked, Sex-Limited, and Sex-Influenced. The first two are most applicable to bird plumages, so I'll stick to those (sex-influenced accounts for male-pattern baldness and other traits that can appear in both males and females but differ in the likelihood or severity of their expression because of how they are affected by hormones).
Sex-Linked: Imagine that there is a tropical bird whose males are all blue and whose females are all green. This could be accounted for a by a gene located on the W and Z chromosomes as follows. The gene for plumage color has two varieties: a recessive gene for blue plumage (I'll call "b") and a dominant gene for green plumage (I'll call "G"). Now further imagine that all W chromosomes have the "b" gene and all Z chromosomes have the "G" gene. Thus males would have bb and females would have bG, resulting in a blue color in males and a green color in females. I don;t know that that this is really how it works in most cases, but it's one explanation. The difference in eye color between male and female bushtits could also be something like this.
Sex-Limited: This is the case that could fit house finches (and according to my reference, accounts for the male peacock plumage). In this case, all house finches have a gene somewhere for "basic" plumage, the brown streaky plumage that juveniles and females have. Both M and F birds have this when they are born. Somewhere else there is another plumage gene that both M and F birds also have, but which is only expressed in the presence of male hormones. So at some point in the male bird's life (bird puberty, if you will), genes on the W chromosomes get busy cranking out male hormones and the red plumage shows up in males, while the females stay brown. This second gene is further complicated by having both dominant and recessive traits (the red and gold varieties). As you can see, many bird species fit this pattern.
Furthermore, I think that plumage changes that occur during breeding season can also be explained by a variation of this. In this case, it would be seasonal increases in hormones that cause the secondary plumage genes to be expressed, where the non-breeding plumage is the "base" plumage and the gene for brighter plumage is triggered by the presence of hormones. In birds where only the males change plumage, the gene is triggered by high levels of male hormones. In birds such as grebes and cormorants, where both sexes change plumage, it is the same process, but the gene for breeding plumage is less selective, and is triggered by higher levels of either male or female hormones.
This is somewhat speculative, since I don't have genetic texts on birds other than the short references described above. Anyone who does, please let me know if these examples work, or if there are alternative explanations. - Teresa Michelsen temi461 at ecy.wa.gov