Op verzoek schreef ik een paar jaar geleden onderstaand stukje.
Voor zover ik nu weet is er nog steeds geen betere DNA studie gedaan en is deze tekst nog actueel (maar ik kan me natuurlijk vergissen).
White or blue, in Ross's or Snow, how come?
Peter de Knijff
On request of the CDNA I review and summarize the most relevant genetic literature in order to reconstruct the – ?rare? - occurrence - of blue-phase Ross’s goose (Anser rossi). The three most relevant sources, which are attached, can be found at the end of this note, in the reference list. I will not individually refer to these papers time after time, simply in order to easy reading and my own time constraints on preparing this note. For some conclusions I have also used sources that are referred to and discussed in these three papers.
1. Blue and white Lesser Snow Goose
First, let’s deal with the two colour forms of the Lesser Snow Geese (LSG; Anser caerulescens). It is very likely that until the late 18th century, the Eastern LSG’s (now harbouring a substantial amount of blue phased birds) and the Western LSG’s (originally being exclusively white) behaved as classical fully allopatric populations, both in terms of breeding and wintering ranges. Both populations likely originate from a single genetic ancestor carrying an mtDNA sequence type B. It is assumed that this ancestor was split in the late Pleistocene, giving rise to two clearly different groups of mtDNA B-type sequences.
Around 1920 (i.e. after at least 300.000 thousands years or more of isolation), both populations came in contact again, especially in the wintering area (coastal Texas and Louisiana), with the contact zone gradually widening allowing more and more mixing among the pure white western with the white/blue mixed eastern birds. Massive change in wintering habitats, and geese becoming adjusted to man-made agricultural pastures allowed the rapid increase of population sizes and admixture between these two populations, and the increase of blue morphs in the western populations.
Important in this respect is the notion of partial assortative mating: blue morphs derived of pure blue morph male and female pairs mate exclusively with other blue morphs, whereas blue morphs from mixed pairs tend to be less specific to a colour form.
As a consequence, during the last 150 years, but more so during the last 80 years, the nearly complete allopatric LSG populations gradually mixed, and this process is still ongoing, resulting in a continuous genetic exchange.
The two colour forms can be nearly fully explained by a single base-pair change in the MC1R gene (this gene is involved in hair- and skin colour regulation in many animals, including humans). This single change, results in an amino-acid change on position 85, leading to a Valine (val) to Methionine (met). Birds with two copies of met-85 are full-blue form, birds with two copies of val-85 are full white form and heterozygote birds can be anything from (nearly) full white to (nearly) full blue.
It is very likely that the first instance of a blue form occurred in an eastern bird, at a time that the two populations (eastern and western) were still fully allopatric. It is also estimated that this mutation could be 380.000 years old (standard deviation ±188.000 years!), but at least after the two LSG populations became allopatric.
Be aware, in reading this, that not all historical sources are accurate. For a long time it was not clear that juvenile / immature pure white LSG have a dark plumage resembling a blue form. It took the American experts quite some time to become aware of this important lack of knowledge! Once solved, old sources became all of a sudden very clear. Also, do not take the dates of splits and mutation very exactly. They are very approximate and based on numerous simplified assumtions.
2. Ross’s Goose
It is generally accepted that prior to ~1970, Ross’s Goose (RG; Anser rossi) wintered exclusively in the San Joaquin Valley in California, thus completely separated from either LSG population. It is frequently reported that since ~1949 RG substantially increased in population size, and shifted their wintering area further east. As a result, the came into contact with wintering and spring migrating LSG’s of either colour form. mtDNA data strongly suggest that originally all RG’s carried an mtDNA type A sequence that was very distinct from the type B sequences of both LSG populations. Moreover, RG’s were exclusively of the white form.
Due to the slow, rare, but continuous interspecific hybridization between RG and LSG since the early 20th century a number of genetic changes happened:
A. The genetic basis for being blue (met-85) was introduced into the RG, by LSG’s).
B. B-type mtDNA sequences were introduced into the RG population by LSG’s and A-type mtDNA sequences were introduced into the LSG’s by RG.
This exchange of genetic information, combined with a rapid population increase of the originally small RG population led to the near randomization of A and B type mtDNA sequences among the two LSG populations and the RG, and, the introduction of blue forms from the eastern LSG into the western LSG and - some time later - the RG population.
3. Conclusions
During the late 19th and early 20th century, the complete allopatric populations of eastern and western LSG and the RG came into contact, especially in the wintering and spring migration staging sites. This resulted in mixing of the two LSG populations, that lead to the introduction of the blue form from the eastern (mixed colour form) population into the western (pure white) LSG population. Also, the genetic basis of the blue form was introduced into the (pure white) RG population. These processes must have involved (limited?) hybridization. If this happens during rapid population expansion (as is the case in all three forms of goose) it can change the original genetic make-up of a allopatric species in all kind of unexpected ways.
As this is exactly what we now see, there is no reason to assume that both LSG populations represent two different species and that the RG is NOT a different species. It is true (or very likely) that the blue-form RG is the result of a relatively recent hybridization and back-cross event between LSG and RG. This has no further bearing on the specific status of the RG, apart from the fact that it presents a unique opportunity for a detailed reconstruction of hybridisation between two species of goose.
At present, at least to my knowledge, mtDNA sequence analyses can not be used to infer the specific status of an unknown goose sample. In addition, there are also no other (autosomal) genetic fragments that could be used as such. This renders these three populations from two distinct goose species a classical example of the inadequacy of DNA to (sometimes) infer species status. In this case, phenotype (i.e. how the birds look like), is by far superior as an identification method.
References.
Mundy NI, et al. Science 2004; 303: 1870 – 1873.
Weckstein JD et al. The Condor 2002 ; 104 : 432 – 436.
Cooke F et al. The Auk 1988; 105: 467 – 479.