SEGREGATION OF T W O ALLELES FOR COLOR OF DOWN I N
PARTHENOGENETIC AND NORMAL TURKEY
EMBRYOS AND POULTS
M. W. OLSENl
AND
E. G. BUSS2
United States Department of Agriculture
and
The Pennsylvania State University
Manuscript received March 20, 1972
ABSTRACT
Data were collected on the down color of 1,386 parthenogenetic embryos
and poults from eggs of 306 young virgin and older nonmated bronze turkey
hens. Each hen involved was heterozygous (Cc) for alleles which affect feather
pigmentation. Six hundred and sixty nine white and 717 colored parthenogenetic embryos and poults were observed, a 1: 1 ratio. Subsequently, 89 of
these 306 heterozygous bronze hens were inseminated with semen from BSW
(cc) males and down color of embryos and poults from fertilized eggs recorded.
the 89 females produced a total of 233 white and 239 colored normal embryos
and poults. The 1:l ratio for down color among both parthenogenetic and
normal embryos and poults shows that, in both instances, the two alleles at a
locus on an autosome segregated equally. Four possible cytological routes
which would lead to diploidy were discussed.
HE vast majority of cells encountered in incubated, unfertilized turkey eggs
Thave been found to be diploid, 2A-ZZ (YAOand OLSEN1955; SATOand KOSIN
1960). This was likewise true of somatic cells of hatched parthenogens (POOLE
1959). The means by which diploidy occurs has been a matter of much interest.
The findings of POOLE
et al. (1963) and POOLE
(1965) that some parthenogens
can be heterozygous at one or more histocompatibility loci led these authors to
conclude that restoration of diploidy occurs after meiosis 11,due to the reuniting
of the second polar body and the haploid egg nucleus.
I n 1966, OLSENpresented limited data on the segregation of alleles for down
color in parthenogenetic embryos from virgin, heterozygous bronze (Cc) turkey
hens. A 1:1 ratio of white to colored embryos was observed. The present study was
conducted to extend the study of segregation for down color of embryos and
poults from eggs laid by the heterozygous, Cc, bronze hens. Comparisons were
made on embryos produced before and after the hens were mated to Beltsville
Small White (BSW) (cc) males.
MATERIALS A N D METHODS
The total experimental population (over a 3-year period) consisted of 461 hens. Two hundred
eighty one of these were young virgins and 180 older nonmated females; all hens were heteroAnimal Science Research Division AFS Beltsville Md. 20705.
Department of Poultry Science, The Pednsylvania )State University, University Park, Pa. 16802.
Genetics 72: 69-75 September, 1972.
70
M. W. OLSEN AND E. G. BUSS
zygous for the Cc feather-color alleles. Eggs yielding one or more parthenogenetic embryos in
which down color could be established were produced by 306 (210 young and 96 old) of 461 hens.
The hens were maintained in individual cages. Daily collections of unfertilized, pedigreed
eggs were placed each evening in a forced-draft commercial type incubator operating at 99X"F.
Eggs were candled on the 10th day of incubation, at which time all clear eggs and those showing
only unorganized growth were removed. Eggs containing live parthenogenetic embryos were
returned to the incubator and candled at frequent intervals thereafter. Dead and abnormally
developed embryos detected at each candling were removed and examined. Down color was
recorded for all dead embryos in which it could be determined and for all parthenogenetic poults
hatched.
Eggs from mated birds were collected each evening and stored at 55°F. for 1-14 days prior to
incubation. Down color of advanced embryos and poults was recorded for each biweekly setting
of fertilized eggs.
RESULTS
Of the 105 young hens involved in tests in 1969, the 72 hens used in 1970, and
the 104 in 1971, 55.2%, 73.6% and 95.2%, respectively, produced one or more
embryos which survived to an age at which down color could be established.
Corresponding numbers and percentages for older bronze hens follow: Fortyseven, 46.8%; 62,58.1%; and 71,53.5% respectively, for years 1969, 1970, and
1971.
Numbers and percentages of white and colored parthenogenetic embryos and
poults encountered during each of the 3 years are shown in Table 1. Data collected for segregation of down color on parthenogenetic embryos and poults in
eggs before and after the hens were mated are presented in Table 2.
To determine if the C and c alleles segregated in a ratio 9f 1:1 during meiosis,
the chi-square test was used. These results indicate there was no significant
deviation from the expected 1:l ratio of white to colored embryos when all data
TABLE 1
Ratio of while io colored parthenogenetic embryos and poulis encouniered in the period
1969-1971 in unfertilized eggs of heierozygous bronze turkey females
Year
1969
1970
1971
Heterozygous
bronze dams
Age
No.
Young1
Old2
Young
Old
Young
Old
58
22
80
53
36
89
99
38
137
Parthenogenetic embryos and poults
__
White (cc)
No. Percent
91.
38
44.3
55.9
13247.1
115
41
47.3
38.7
156
328
53
381
50.5
49.1
50.2
Colored (CC+Cc)
No. Percent
118 55.7
30 44.1
148 52.9
128 52.7
65 61.3
-~
193 55.3
321 49.5
55 50.9
376 49.7
~
1 y r old.
2 y r s and older.
* Significant chi square (P = .%).
a. Total chi square; b. Pooled chi square; c. Heterogeneity chi square.
2
a
59.03
19.29
63.74
29.75
~
118.40
32.71
~
2
b
2.72
.94
__
.914
,695
5.434*
3.923*
.0755
,037
_
.033
C
56.31
18.35
.___
63.05
24.34
_
118.33
32.67
_
_
-
Young
Old
Young
Old
1970
1971
100 49.5
233 49.4
31
89
-
35
21
10
_ _
-
2.3
25
IO
39 51.3
79 49.4
19 55.9
-~
98 50.5
78 48.1
24 60.0
-~
102 50.5
239 50.6
_ _ _
37 48.7
81 50.6
44.1
-15 _
96 49.5
84 51.9
16 40.0
- _
36 53.7
3 33.3
.-
31 46.3
6 66.6
21
2
-
No. of
dams
2
1
2
b
C
.02
23.103 .02
39.64
23.083
39.62
29.182 .053 29.130
a
1 yr.
2 yrs and older.
a. Total chi square; b. Pooled chi square; c. Heterogeneity chi quare,
All birds
Young'
Old2
1969
Year
Age of
dams
Prior to mating
Embryos and poults
White
Colored
(cc)
(CCfCc)
No. Percent
No. Percent
127 47.4
169 50.4
658 52.9
- _ _
166 49.6
586 47.1
- -
116 47.9
50 53.8
2864.6.2
33353.6
126 52.1
43 46.2
232 44.8
54 53.5
I44 48.0
17 _
53.1_
_
286 55.2
47 46.5
141 52.6
15 4.6.9
-~
156 52.0
After mating to BSW males
Normal embryos and poults
White
Colored
(cc)
(CC)
No. Percent
No. Percent
.48
b
26.407
C
18.966
.0268 18.939
45.04.9 3.568 41.481
26.887
a
Numbers of white and colored parthenogenetic embryos and poults encountered among fertilized eggs of heterozygous bronze turkey females prior
to mating and from fertilized eggs of the same bronze females after they were mated to Beltsville Small White ( c c ) males
TABLE 2
4
s
2c3
cn
E
3
0
B
0
BZ
z
+P
t
i
72
M. W. OLSEN AND E. G. BUSS
were pooled for the 3 years. Within years, there was a significant deviation for
the old hens used in 1970, but there was no significant difference when the data
from the old hens were pooled for the 3 years. Although a few individual hens
contributed heavily to the total chi-square values (their progeny did not fit the
1:l ratio of white to colored) for each group, the heterogeneity chi-square test
result indicates that the groups were reasonably homogeneous. Data presented in
Table 1 are therefore interpreted to confirm the finding of a 1:1 ratio of white
to colored parthenogenetic embryos and poults reported previously (OLSEN
1966).
The frequency of white to colored parthenogenetic embryos and poults in
unfertilized eggs of individual hens may be of some interest. Records of hens
shown in Table 1 were analyzed on an individual basis and the percentage of
their embryos and poults having white down calculated. A total of 306 virgin and
non-mated, heterozygous (Cc) bronze female were involved. These data revealed
that 51 of the 306 hens (16.7%) gave rise to parthenogenetic embryos and poults
all of which were colored. Forty-six of the 306 hens (15.0%) gave rise to embryos
and poults, all of which had white down. Each of the remaining 209 hens
(68.3%) gave rise to both white and colored embryos and poults. As expected,
the proportions of white to colored embryos and poults varied with hens. Numbers of hens and percentages of their parthenogenetic embryos and poults with
white down follow:
Heterozygous (Cc)
Bronze Hens
(Number)
8
23
35
19
54
41
17
12
Embryos and Poults
with white down
(Percent)
10.0-19.9
20.0-29.9
30.0-39.9
40.0-49.9
50.0-59.9
60.0-69.9
70.0-79.9
80.0-89.9
Data on down color of embryos and poults from eggs of the 23 hens inseminated
in 1969, the 35 females inseminated in 1970, and the 31 hens inseminated in 1971
are shown in Table 2. Data presented show that segregations of white and colored
embryos in eggs obtained before and after insemination of the same heterozygous
bronze hens fit a 1:1 ratio. Pooled chi-square values shown in Table 2 indicate
that alleles determining white or bronze down color segregated independently
and randomly during meiosis, both in unfertilized and fertilized eggs.
DISCUSSION
The data presented appear to indicate that the C and c alleles occurred equally.
et al. (1971) that the origin
These data lend support to the suggestion of DARCEY
of parthenogenesis was likely to be from haploid eggs. However, there are a
PARTHENOGENESIS IN TURKEY
73
number of cytological routes which turkey germ cells might follow in the establishment of diploidy and still give an apparent 1:1 segregation.
BEATTY(1957) lists several such routes as follows: (1) Suppression or re-entry
of the first polar body followed by meiosis 11. (2) Suppression or re-entry of the
second polar body. (3) A nuclear division at mitosis I in the absence of a corresponding cytoplasmic division. (4) Fusion of two haploid mitotic products. Suppression or re-entry of the first polar body in the case of birds should, theoretically, give rise to some female parthenogens. Data now available make it possible
to dismiss route 1 on the basis of sex of turkey parthenogens. All fully developed
parthenogenetic embryos examined to date have been males (POOLE
and OLSEN
1957). This has likewise been found true of those parthenogens that have survived to maturity (OLSENand MARSDEN
1954a; OLSEN1965a).
Suppression or re-entry of the second polar body (route 2) is a logical route by
which diploidy could be restored. Polar bodies in eggs produced by chickens and
turkeys come to lie just beneath the vitelline membrane along the upper surface
of the germinal disc and in close proximity to the egg nucleus (OLSEN1942) ,and
they are never completely extruded from the ovum.
A parthenogen could be heterozygous at a given locus if crossing over occurred
between the centromere and that locus, and if the 2nd polar body was not
extruded (route 2). But this does not exclude an apparent 1:l ratio because this
crossing over would be hard to detect if the locus were close to its centromere.
Thus when data for all birds shown in Table 1 are considered, it can be calculated
that there was an excess of 3.4% colored parthenogenetic embryos. Making the
assumption that these should be assigned to a heterozygous category resulting
from crossing over, examination of Figure 2 in the paper by NACEet al. (1970)
reveals that the gene f o r bronze plumage color in turkeys is located approximately
2 map units from the centromere. If the locus is, indeed, that close to the centromere, the apparent 1:l ratio observed here would not be incompatible with a n
explanation involving route 2. By this mechanism, an autosomal diploid equivalent to a fertilized egg would be produced. The unfertilized turkey ovum, therefore, would start mitotic division as a diploid cell. Evidence that cell divisions are
not normal and result in the production of some haploid, tetraploid and heteroand Buss (1968)
ploid cells has been shown by SATOand KOSIN(1960) ,DARCEY
and SARVELLA
(1971). This is not surprising, however, since the type of parthenogenetic development encountered in eggs of both chickens and turkeys from its
onset, shows every indication of being abnormal. Large yolk-ladened blastomeres
become arranged in multiple layers and form compact masses. Necrotic areas and
vacuoles are encountered frequently throughout the protoplasmic disc. Many of
the cells encountered in the blastoderm of newly laid unfertilized eggs show little
affinity for iron hematoxylin stain, indicating that they are either moribund or
dead (OLSENand MARSDEN
195413; HANEY
and OLSEN 1958). Cell division
occurring in such an unfavorable environment could easily deviate from the
normal pattern. The surprising thing is that cells found in such a state of disarray
will resume development; and; in rare instances give rise to normal parthenogenetic embryos. Cytological events which make this possible take place within
74
M. W. OLSEN A N D
E. G . BUSS
the incubator, at which time, some of the more viable blastomeres composing
these cell masses resume development and eventually form an entirely new,
single cell-layered blastoderm from whence develops the future embryo. Construction of a new blastoderm, however, requires time, thus the underlying reason
for the characteristic 2-3-day delay regularly observed in the onset of parthenogenetic development (OLSEN1965b).
If nondisjunction occurred at meiosis 11, the sex-chromosome condition of the
resulting diploid cell would be either ZZ (male) or YY (female?). In Drosophila,
1954). The absence of
the YY constitution has been found to be lethal (STALKER
female parthenogens indicates that the YY constitution is likewise lethal in the
case of turkeys.
Restoration of diploidy at or following mitosis I (routes 3 and 4) should give
rise to individuals homozygous for all loci. However, some parthenogens, as
already noted, have been shown to be heterozygous at one or more loci controlling
histocompatibility (POOLE
et al. 1963; POOLE
1965), and at the locus controlling
bronze plumage color (OLSEN1966). These observations were major considerations in leading the aforementioned authors to conclude that cytological route 2
is the one being followed by unfertilized turkey ova in restoration of diploidy.
Strong cytological evidence may be cited in support of the contention of POOLE
et al. 1963; POOLE
1965; and OLSEN1966 that diploidy in unfertilized turkey
eggs occurs due to suppression of the second polar body. Studies conducted by
OLSEN(1942) and OLSENand FRAPS
(1950) with ova of the domestic fowl and
by OLSENand FRAPS
(1944) with eggs of domestic turkeys revealed that the
initial stages in the formation of the second polar body take place in the ovary
just prior to ovulation. I n mated birds the ovum, upon entering the infundibulum, is fertilized, usually within 15 minutes. Penetration of the ovum by the
sperm provides the necessary stimulus for completion of meiosis I1 (OLSEN1942;
OLSENand FRAPS
1944).
The important question is: What happens when no sperm are present to provide the stimulus required for completion of meiosis II? Available evidence indicates that, in the absence of sperm, chromosomes of the second polar body and
those of the egg nucleus probably would not separate. This view is also shared by
WATERMAN
(1948) ; RUGH (1964) and BEATTY(1967) among others.
Cytological route 2 to diploidy provides an explanation as to why some parthenogens can be heterozygous at certain loci. The data presented in this paper
show that alleles at one locus segregate in a 1:l manner. However, it will not be
known if all parthenogens arise from eggs that have not extruded the second polar
body and are, therefore, 2n, until other studies using other loci more distant from
centromeres on autosomes are reported.
The author wishes to thank Dr. BERNARD
WEINLAND,
Biometrical Services, ARS, Beltsville,
Maryland, for the statistical analyses.
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Academic Press, N. Y.
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75
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K. M. and E. G. Buss, 1968 On the origin of the diploid number of chromosomes in the
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