Fundamental questionsWhich can only be answered by the scientific communityThere remain a number of questions to which aviculturalists have sought definitive answers for many years. These are questions requiring a level of investigation quite beyond the scope of even the most expert of ordinary breeders. The fact that answers are not forthcoming attests to the difficulties which have been encountered even by the scientific community with all the resources at its disposal. Those resources, however, continue to expand and be refined at an almost alarming rate, to the extent that previously intractable problems are now routinely overcome. Are answers to these questions already buried away somewhere in the scientific literature; or is it a matter of waiting for the right individuals or teams see the value in taking up a particular challenge? Here are some of these questions:
•
How many chromosome pairs in parrots?This question has to be presented and answered carefully to avoid any confusion. The answer we usually expect is that posed in the above heading, but that is not the way professional geneticists normally express the chromosome compliment of any species since they have to allow for a variable we do not often consider. That variable is known as ploidy and refers to the number of sets of chromosomes present in a species. We are used to dealing with birds which, like most higher organisms, have two sets of chromosomes (each parent contributing one set at the time of conception) and such species are known as diploid species. [This is not universal, and plants in particular may have multiple sets of chromosomes.] Confining our attention to diploid species such as birds, the chromosome number can be expressed in two ways:
We tend to think in terms of the number of different chromosomes present (or the number of pairs) and this corresponds to the haploid number. Geneticists, bearing in mind ploidy, usually express the chromosome compliment of a diploid species in the form 2n = X , where n is the haploid number and X the total number of chromosomes present in somatic (body) cells. Humans for example have a haploid number of 23, but a diploid number of 46, and it is essential to know by what convention any chromosome numbers quoted are expressed. •Genetics for Budgerigar Breeders by Taylor and Warner is usually the first port of call for aviculturalists when seeking answers to genetic questions. The authors make the point that budgerigars, besides having a number of chromosomes of normal size (the macrochromosomes), also have a number which are a great deal smaller and correspondingly difficult to count (the microchromosomes). The numbers are given as 13 pairs of macrochromosomes together with up to 18 or more microchromosomes; giving a total of plus or minus 31 pairs (the haploid number). No source is given for this information and we have to assume that it was either Dr L Auber, c.1941 or, even earlier, Prof F A E Crew, c.1935. On his Avian Genetics page Anthony Olszewski points out the problems which arise in determining the chromosome count of birds due to the presence of relatively large numbers of microchromosomes. Noting that few attempts have been made to karotype birds he does, however, quote figures for three species: the budgerigar, the canary, and the pigeon. No source is given for these figures and in a personal communication Anthony was unable to recall where the information came from. The figures given are (plus or minus in each case) 58 for the budgerigar, 80 for the canary, and 80 for the pigeon. Ploidy is not stated, but comparing the figure given for the budgerigar with that supplied by Taylor and Warner suggests that it is the total chromosome count or diploid number. These figures must be halved to give the number of different chromosomes or haploid number. Because of its economic importance the genome of the domestic chicken or fowl (Gallus gallus) has been more widely and intensively studied than any other avian species. One of the main centres for this research is The Roslin Institute. It is of some interest that the forerunner of the Institute, the Department of Research in Animal Breeding, at Edinburgh University, was formed in 1920 and was headed by none other than Prof F A E Crew. An article at the National Academies pages, The Chicken Gene Map, outlines the worldwide cooperative efforts now being undertaken to map the chicken genome. Here, the chicken is quoted as having a diploid number of 2n = 78 of which 60 are classed as microchromosomes. This equates to a haploid number of 39, comprising 9 macrochromosomes plus 30 microchromosomes. The ZW (XY) pair is included in the macrochromosome total of 9. You can see a chromosome spread of a female chicken at the Molecular and Biology section at UC Davis (foot of page). On average, microchromosomes are about a tenth the size of macrochromosomes. No firm conclusions can be drawn from the foregoing: even for the chromosome compliment of the budgerigar, let alone parrots in general. For the present the figures presented by Taylor and Warner do not seem to have been superceded by more recent research. It is evident that the chromosome counts of different avian Orders (27) vary quite widely but whether, and to what extent, this extends into the different Families and even species is not yet clear. •
What is the nature of psittacin pigment?Aviculturalists have long been aware that the yellow, through orange, to red pigments found in the plumage of parrots are different to the similarly coloured pigments found in other commonly kept birds. The name psittacin was coined to make clear the distinction between the pigments found in parrots and the lipochromes (carotenoides) found in, say, the passerines. Because birds along with all vertebrates are unable to synthesize carotenoides within their own biosystems these compounds must be ingested in the diet. •
Ultraviolet reflectance•
The budgerigar cere• |
|
Main Articles index • Other Sites index • Books • Please take me Home http://birdhobbyist.com/parrotcolour e-mail: ClveHesford@aol.com |