Download The Genetic Basis of Color Differences Observed in the Mute Swan

504
GeneralNotes
[Auk, Vol. 85
in August,was a bird capturedin the Bay of Panama off Balboa, some8 degrees
north of the equatorin 1937 (Murphy, 1938).
LITERATURE CITED
ALEXAI•rDER,
W. B. 1928. Birds of the ocean. New York, Putnam.
H^RTERT,E.
1912-21. Die VSgel der pal[tarktischenFauna, vol. 2. Berlin, Fried-
lander & Sohn.
J^•fESO•,W.S. 1958. The WanderingAlbatross. Londo.n,Hart-Davis.
JONES,F. W. 1936. The wanderingof albatrosses.Emu, 36: 103-105.
MuRr•q¾, R. C.
Mus. Nat.
MuurH¾, R. C.
40:
1936. Oceanic birds of South America, vol. 1. New York, Amer.
Hist.
1938. The Wandering Albatross in the Bay of Panama. Condor,
126.
Om;^>:DO,C. 1958. Cattura di un Albatro urlatore, Diomedea exulans exulans
(Linnaeus) in Sicilia. Rivista Italiana Orn., 28: 101-113.
PE^XA•;•;,D. B. 1960. Tropical records of the Wandering Albatross in the Atlantic
Ocean. Ostrich, 31: 105.
VAN OOaDT,G.J.
1939. Over de noordelijkste verspreidingsgrens
van enkele antarctische zeevogelsoorten
in den AtlantischenOceaan. Ardea, 28: 1-4.
ROBERTO. PAXTOIq,Setauket, New York 11785.
The genetic basis of color differences observed in the Mute Swan (Cygnus
olor)L--The downyyoungof the Mute Swanappearin either of two color phases,a
white or gray. The white phase was first reported in 1868 among the swans on
Lake Geneva, Switzerland (Hilprecht, Hocherschwan, Singschwan, Zwergschwan,
Wittenburg Lutherstadt, Die Neue Brehm-Bucherie, 1956); it is usually less common
than the gray phase,but becomesmore frequentas one moveseastwardacrossEurope,
and it is sometimesreferred to as the Polish variety. Cygnets with gray down have
slate-gray bills and feet; the white phase birds have tan colored bills and feet. The
foot color of eachphasepersistsin the otherwiseidenticalwhite-plumagedadults, and
may be usedto determineits down color phase. This study was undertakento determine the geneticbasisof the color phase.
Methods
and materials.---In
1963 the State Division
of Conservation initiated
a
bandingprogramto determinethe statusof the Mute Swan in Rhode Island. Nesting
sites were watched and location and down color were recorded by the senior author.
In the 1967 hatching season53 nestingsites were kept under observation. Cygnet
data were obtained from only 36 nests; 11 were lost by flooding during adverse
weather and 6 were destroyedby predation.
Color phase was observed as soon as possible after hatching to reduce any effect
of possibledifferential mortality. Unhatched eggs were broken and color phase
determined on embryos far enough advanced to exhibit down color.
Adult color phase was determined for all nesting pairs, and additional adult data
were obtained from previous banding records.
Resultsand dlscussion.--The1967 nestingcolor phasedata summarizedin Table 1
indicatesthat color differenceis determinedby a singlegene and that the gray allele
is dominant
to white.
Examination of sex data indicated a preponderanceof white females. As birds
follow an XO methodof sex determination,the male being homogameticXX or hayContribution number 1255 of the Rhode Island Experiment Station.
July, 1968]
General Notes
TABLE
1967 MAT•C
Typeof mating
No. of adults
505
1
DAT•
No. of cygnets
Total
Gray
White
Gray
White
individuals
Gray X gray
Gray X white
58
5
0
5
145
18
205
40
White X white
0
4
0
2
12
12
16
ing two sexchromosomes,
and the femaleheterogametic
XO or having onesexchromosome, a sex-linked recessivewould result in a greater number of visible recessives
in females.
Banding data from previous years included 488 swans whose sex and color phase
had been recorded. Of the 288 males, 29 or 10 per cent were white phase; of the 200
females,51 or 26 per cent were white phase.
If' in a random mating populationwe let the frequencyof the dominant allele (A) of
an autosomalgene he p and the frequency of the recessiveallele (a) be q, then the
frequencyof the three genotypesare:
p2AA, 2 pq Aa and q2aa
If a geneis sex-linked,the frequencyof the genotypeswithin sexeswill differ. In
the caseof the male, which is homogametic(XX) the ratio of the recessivegenotypes
to the total numberof individualsrepresents
q2,just as if the genewere autosomal.
In the caseof the female,which is heterogametic(XO), there can be no heterozygotes
(Aa) and the possihlegenotypicfrequenciesare p AO and q aO and the ratio of recessives
to the total numberof individualsrepresents
q.
51
If the geneis sex-linked,the estimatedvalue of q basedon femalesis q•-
2OO
--
.255. An estimatebasedon malesand the assumptionof random mating is q• •
2•2•8:
.316.
The
estimates
qfand
q•do
not
differ
by
astatistically
significa
amount and matingscan be assumedto be at random. The estimatedvalue of q for the
populationthen can be a weightedestimateof qf+• m .293.
Further evidenceof sex-linkagewas obtained by examiningfamilies that would
yield a sex-linkedresult, i.e. recessive(white) male X dominant (gray) female or the
male contributingthe sex chromosomecarrying the recessiveallele to his daughters.
Only two suchfamiliesappearedin the 1967 matings;theseyielded9 malesand
7 females.All male cygnetswere gray and all females(XO) were white, obtaining
their sex chromosomefrom their recessive(white) sire. This clearly indicates the
color phasesto he sex linked.
Summary.--Examinationof bandingdata and parentsand progenyat nestingsites
of the feral populationof Mute Swansin RhodeIsland indicatedthat the gray and
whitecolorphasesegregating
in the populationwas due to a singlesex-linkedgene.
The gray wasdominantto white and the genefrequencyof the recessive
allele (white)
was estimated to be .293.
Acknowledgements.---The
authors greatly appreciatethe assistanceof the Rhode
Island Departmentof National Resources,
Division of Conservation,in someof the
data collectionphaseof this study.--R. E. MunRo, L. T. S•a,
and J. J. KuP^,
University of Rhode Island, Kingston, Rhode Island 02881.