The Roles of Polyembryony and Embryo Viability in the Genetic System of Conifers
Author(s): Frank C. Sorensen
Source: Evolution, Vol. 36, No. 4 (Jul., 1982), pp. 725-733
Published by: Society for the Study of Evolution
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Evolution, 36(4), 198 2, pp. 72 5-733
THE ROLES OF POLYEMBRYONY AND EMBRYO VIABILITY IN THE
GENETIC SYSTEM OF CONIFERS
FRANK C. SORENSEN
Pacific Northwest Forest and Range Experiment Station, 809 NE 6th Ave., Portland, Oregon 97232
Received July 14, 1980.
Revised Februa ry 6, 1981
The mating and regeneration habits of
many important coniferous species seem
conducive to self pollination, exchange of
pollen among relatives in small family
groups, and relatively high levels of in­
breeding. Most species show large in­
breeding depression in growth, and the
frequency of self seedlings in wind-polli­
nation progeny is generally low. In this
paper, I will show that low embryo sur­
vival plus the presence of more than one
embryo in a single ovule (polyembryony)
allows considerable post-fertilization em­
bryo abortion and selection without com­
parable wastage of ovules. In combination
they form an important mechanism for
maintaining heterozygosity as well as local
adaptation .
Self incompatibility, in the sense of
blockage to pollen tube penetration and
selective fertilization (Squillace and
Bingham, 1958), could perform a similar
function. It also would contribute to
maintaining heterozygosity in the presence
of high frequencies of self pollination. Ex­
perimental evidence for self incompatibil­
ity in conifers is lacking (Hagman and
Mikkola, 1963; Hagman, 1972) but it can
not be ruled out at this point. If selective
fertilization occurs it could further aug­
ment the effect of polyembryony.
Low viability of inbred embryos, a fea­
ture of many of the widespread coniferous
species, is attributed to recessive lethal
and deleterious alleles which become
homozygous after inbreeding (Orr-Ewing,
1957; Sarvas, 1962 ; Hagman and Mikko­
la, 1963; Mergen et al., 1965). Many of
these alleles apparently are present at low
frequencies in the breeding populations
(Koski, 1971). Cytological examinations
have shown embryo abortion to occur ear­
ly in embryogeny after apparently normal
pollen germination and fertilization (Orr-
Ewing, 195 7; Hagman and Mikkola,
1963; Mergen et al., 1965 ; Sarvas, 1968).
Polyembryony, a rare derivative phe­
nomenon in angiosperms but unusually
common in gymnosperms (Chamberlain,
1966, p. 348), is of two types. One is cat­
egorized as simple, archegonial, polyzy­
gotic, or non-cleavage, and the other as
monozygotic or cleavage . Archegonial
polyembryony, characterized by indepen­
dent fertilization of more than one arche­
gonium within an ovule, gives rise to het­
erogenic embryos within the ovule.
Within the family Pinaceae, it occurs in
the genera Larix, Picea, and Pseudotsuga
(Dogra, 196 7, p. 17). Monozygotic po­
lyembryony is characterized by cleavage
of the embryo and gives rise to isogenic
embryos. Both mono- and polyzygotic po­
lyembryony occur in the genera Pinus and
Tsuga (Dogra, 196 7, p. 17, 83) . The sit­
uation is not clear in Abies, which is clas­
sified as having non-cleavage polyem­
bryony by Dogra (1967, p. 16); but in
Abies amabilis at least there appear to be
more embryos than could arise from non­
cleavage polyembryony alone (Owens and
Molder, 1977), which indicates a pattern
like Pinus and Tsuga.
Polyembryony eliminates the effects of
aborted embryo development when the
abortion occurs in an ovule that contains
an additional functional embyro to replace
the degenerating one. Also, because one
embryo usually ends up dominant in the
embryo cavity, polyembryony provides
opportunity for selection between embryos
of unequal vigor during early embryo de­
velopment. Embryological development
can be climatically as well as genetically
disturbed. Monozygotic polyembryony
has the potential for reducing the effects
of climatic disturbance; archegonial poly­
embryony has the potential for reducing
725
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726
F. C. SORENSEN
either climatic or genetic disturbance.
This paper is concerned with genetic dis­
turbances and will describe the role of ar­
chegonial polyembryony in reducing the
effect of these disturbances.
Two contrasting examples will be pre­
sented. One is based on data from coastal
Douglas-fir (Pseudotsuga menziesii var.
menziesii), a subspecies with an extensive
natural range, nearly complete intermin­
gling of micro- and megasporangiate stro­
bili on the crown, inbreeding depression
of growth (Sorensen and Miles, 197 4), and
severe inbreeding depression of embryo
survival (Sorensen, 1971). The second is
based on data from noble fir (Abies pro­
cera), an upper-slope species of the Cas­
cade Ranges with very little intermingling
of micro- and megasporangiate strobili,
inbreeding depression of seedling growth
comparable to Douglas-fir, but relatively
slight inbreeding depression of embryo
survival (Sorensen et al., 1976).
6) the probability of an embryo being vi­
able depends exclusively on whether it
results from self or cross fertilization.
Any effects of less close inbreeding, cli­
matic and insect damage, etc. are not
included in the calculations.
An ovule contains a sample of n em­
bryos drawn at random from the total em­
bryo production of a tree. These embryos
can be placed into four classes (viable self
embryo, nonviable self embryo, viable
outcross embryo, nonviable outcross em­
bryo) with probabilities P 1 , P 2 , P 3 , and
P4,
Class
Self viable
nonviable
Outcross viable
nonviable
Probability
P1
=aX
P2
= (1
(1)
- a)X
(2)
P 3 = c(1 - X)
(3)
P4 = (1 - c)(1 - X) (4)
where X is the probability an embryo is
the result of self fertilization and (1 - X)
is the probability an embryo is the result
of cross fertilization, a is the conditional
METHODS
probability that the embryo is viable given
The mathematical calculations are that it resulted from self fertilization and
modified from Lindgren (1975). Four (1 - a) is the conditional probability that
classes of embryos (viable and nonviable it is not viable, c is the conditional prob­
self, and viable and nonviable outcross) ability that the embryo is viable given that
and two classes of pollination (self and it resulted from cross fertilization and
outcross) are designated. Other levels of (1 - c) is the conditional probability that
inbreeding could be handled . However, it is not viable.
most of the available experimental data on
The parameters a and c can be esti­
inbreeding in conifers are for selfing, so mated from filled and empty seed propor­
this presentation will be limited to the tions after controlled pollinations. The ex­
contrast between selfing and outcrossing. pected proportions of round normal
Proportions of self and cross pollination appearing seeds which are filled, Sa (i.e.,
and embryo viabilities are related to fre­ seeds containing at least one viable em­
quency of self seedlings based on the fol- . bryo after self pollination), are related to
lowing assumptions:
the viability of selfed embryos, a, by the
expression
1) ovule development is not affected by
class of pollination (self or outcross),
Sa = 1 - (1 - a)"
2) n egg cells in an ovule are fertilized by
where n is the number of embryos per
pollen and produce n embryos,
ovule. An estimator of a is then
3) the final product of each ovule is one
= 1 - "\V1 -Sa.
(5)
seed with zero or one embryo,
4) a seed is empty if all the n embryos are
Similarly, an estimator of c based on out­
nonviable,
cross data following control pollination is
5) all seeds with viable embryos can ger­
minate, and
C = 1 - "\V 1 - Sc
(6)
a
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727
POLYEMBRYONY AND SELFING IN CONIFERS
where, Sc is the proportion of round nor­
mal appearing seeds containing a viable
embryo after cross pollination.
The particular array of embryo classes
occuring among the n embryos of an ovule
is multinomially distributed such that
P(N 1 = n 1, ... , N 4
= n 4)
n!
. PI" '
nl! . .. n4!
• · •
P4"•
where N 1 , N 2 , N 3 and N 4 are the num­
bers of each respective embryo class and
Nt + N 2 + N 3 + N 4 = n, the number of
embryos per ovule .
Ovules can contain any combination of
the four classes of embryos and can be
identified according to the presence of vi­
able self and/or viable outcross embryos.
Four ovule classes of particular interest
are:
Class 1-(N 1 > 0, N 3 = 0) ovules contain
at least one viable self embryo
and no viable outcross embryos.
Class 2-(N 1 > 0, N 3 > O) ovules contain
at least one viable self and one
viable outcross embryo.
Class 3-(N 1 = 0, N 3 > 0) ovules contain
no viable selfs and at least one
viable outcross embryo.
Class 4-(N 1 = 0, N 3 = O) ovules contain
no viable selfs and no viable out­
cross embryos.
Recognizing that the probability an ovule
is viable is P 1 + P 3 (1) and (3) , the prob­
abilities of the four above events are cor­
respondingly:
Class 1-P(N1 > 0, N 3
= (1 - P3)"
=
0)
Pt - P3)"
Class 2- P(N 1 > 0, N 3 > O)
1 - (1 - Pt)"
- (1 -
(7)
- (1 - P3)"
(1 - Pt - P3)"
(8)
+
Class 3-P(N1
= (1
= O,
N 3 > O)
- Pt)"
- (1 - Pt - P3)"
Class 4- P(N2 = 0, N 3 = O)
= (1 - Pt -' P3)"
(9)
(10)
and the proportion of filled seeds is
P(N1
+ N3 >
1 - (1 -
0)
=
P1 - P3)".
(11)
If viable self embryos are always se­
lected against when they occur in the same
ovule with a viable outcross embryo, the
probability that a seedling in the progeny
is the result of self fertilization is
P(NI > 0, N 3 = 0)
P(NI + N3 > 0)
which can be related to the probability of
self pollination and embryo viability.
If viable self embryos are not always
selected against when they occur in the
same ovule with a viable outcross embryo,
values for relative competitive abilities
must be assumed. For example, if n = 2
and the strobili receive self and outcross
pollen , some ovules will contain both one
viable self and one viable outcross em­
bryo. Solving (8) for n = 2 shows that the
probability of this occurring is 2PdJ 3. If
self and cross embryos are equally com­
petitive and change determines which one
fills the embryo cavity, the probability
that the self embryo is successful is
.5(2PdJ3) = PdJ 3. If viable self embryos are
less competitive than viable outcross em­
bryos, for example they fill the embryo
cavity only 40% of the occasions when
both occur in the same ovule, the proba­
bility that the self embryo is successful is
.4(2PdJ 3) = .8PtP 3 . If the above values
are reversed and self embryos favored, the
probability that the self embryo is suc­
cessful is .6(2P tP3) = l.2P 1P3 · Higher
levels of n may be handled in a similar
manner, paying particular attention to the
numbers of viable self and outcross em­
bryos competing in the same ovule.
Values of n probably depend on species­
specific characteristics of the pollination
and fertilization mechanisms, the amount
of pollen in the air , and pollen viability.
Coastal Douglas-fir has four to six arche­
gonia per ovule (Allen and Owens, 1972,
p. 110). An average of 1. 8 elongated pol­
len grains (maximum of nine) have been
counted in the micropylar canal (Silen ,
1978, p. 8). There are no published reports
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728
F. C. SORENSEN
on noble fir; but Abies amabilis, a related
western species, contains two or three
archegonia in the mature female gameto­
phyte (Owens and Molder, 1977). Orr­
Ewing (195 7, Tables 3 and 5) counted av­
erages of 2.1 and 3. 3 embryos per ovule
for two coastal Douglas-fir trees after con­
trolled pollination . Several embryos per
ovule were observed in Abies amabilis,
but the number of polyzygotic embryos
would be limited to two or three, the num­
ber of archegonia (Owens and Molder,
1977). Another species with only polyzy­
gotic polyembryony, Picea abies, has the
potential to originate three embryos in
about 90% of the ovuleii (Sarvas, 1968);
but even in a year of abundant pollen pro­
duction, the average was 1. 9 embryos per
ovule. Like Douglas-fir, the actual pro­
duction appeared to be less than the po­
tential. Pinus sylvestris, another species
combining both mono- and polyzygotic
polyembryony, is reported to average 1. 7
fertilized archegonia per ovule, with the
number of fertilized archegonia being re­
lated to pollen abundance (Sarvas, 1962).
Based on these observations, values of
n = 1, 2, and 3 seemed adequate to cover
most cases of wind pollination.
With more than one embryo per ovule,
competition between embryos is possible,
but its occurrence and extent has been a
disputed issue. Dogra (1957, p. 15, 84)
briefly presents the earlier ideas. More re­
cent reports generally seem to indicate
that pregermination selection can occur
(Sarvas, 1962; Fowler, 1964; Allen and
Owens, 1972; Owens and Molder, 1979).
Inferential evidence for selection against
viable self embryos when self and outcross
embryos are in the same ovule might come
from the inbreeding depression found in
early growth of many coniferous seedlings
(Franklin, 1969), including Douglas-fir
(Sorensen and Miles, 197 4) and noble fir
(Sorensen et al., 1976). In evaluating the
relationships, two alternative models will
be examined: no embryo selection (chance,
based on the proportions of viable em­
bryos which are self and outcross, deter­
mines which embryo fills the cavity) and
complete selection in favor of the cross-
1.0
- - n=1
;:
- - n = 2 , chance
o.g
=~-= ~ ~ ~~!~~:5
-
-- -
WIOS
n = 3, ou tcross w 1ns
0.8
(f)
~ 0.7
::J
0
~
0.6
lL
ui
(f)
0.5
z
Q 0.4
ti:
g
a:
03
a.
0.2
0.1
0.9
1.0
PROPORTION SELF POLLINATION
FIG. 1. Relationships between proportions of self
pollinations and the expected proportion of self seed­
lings in the wind-pollination progeny of coastal
Douglas-fir and noble fir, given n (level of polyem­
bryony) = 1, 2, and 3. Median species values have
been used for embryo viabilities. The curves are
based on equations 7-11. Values for a and c were
calculated from equations 5 and 6. For the cases
when viable self and outcross embryos both occur in
one ovule, curves are given for two outcomes: either
the outcross embryo always fills the embryo cavity
or chance determines whether self or outcross em­
bryo fills the cavity. Further explanation in text.
pollination embryo when it occurs in the
same ovule with a viable self embryo.
RESULTS
In the present examples, Sc for Doug­
las-fir is 0 . 75 (standard deviation 0.19)
and for noble fir 0.56 (SD 0.15), Sa for
Douglas-fir is 0.06 (SD 0.077) and for no­
ble fir 0.35 (SD 0.14). These are median
values for 35 Douglas-fir and 10 noble fir
trees reported in Sorensen (1971) and So­
rensen et al. (1976). From equations 5 and
6 and selected levels of n the values of a
and c can be estimated.
In Figure 1, the relationships between
proportion of self-pollination (the letter X
in (1)-(4)) and the expected proportion of
self seedlings in the progeny from mixed
self + outcross pollinations are shown for
the two models and for n values ranging
from 1 to 3. These are representative
curves based on the median self- and
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729
POLYEMBRYONY AND SELFING IN CONIFERS
1.0
cross-fertility values given above. Individ­
ual tree curves could depart considerably
from them. As mentioned above, coastal
\,
0.9
Douglas-fir represents a species in which
the viability of self embryos generally is
'\,
0.8
very low; noble fir is a species in which
\\
1-~
the viability of self embryos on the aver­
\'~\
~<~"~-9'
age is not much lower than the viability
~
0.7
'\\
of outcross embryos. The relationships (/)
~
0
between the proportion of self pollination UJ
~\
and the proportion of viable seeds are pre­ UJ
(/)
0.6
~
sented in Figure 2 for the two species. UJ
...J
co
~(;> ~\
The proportion of viable seeds is influenced <t:
~(ll ~\
> 0.5
only by n, not by the model used.
"'~ ~\
\
z
Table 1 gives the proportions of self pol­ 0
i=
'\,
lination and filled seeds associated with 0a: 0.4
'\
2.5%, 5.0%, and 10.0% self seedlings in a.
0
the progeny-given n = 1, 2, and 3 and a:
a.
\
0.3
two levels of embryo competition in poly­
\
embryonic ovules. This is the approximate
range of mean self-seedling proportions
\
0.2
found in the wind-pollination progenies of
\
--n=1
various coniferous species growing in nat­
~
--n=2
ural stands (Sarvas, 1962; Squillace and
- - - - n=3
0.1
Kraus, 1963; Franklin, 1968; Sorensen,
1973; Koski and Malmivaara, 1974; Miil­
0
ler, 1977).
0.4
0.6
0.8
1.0
0
0.2
Figure 1 and values in Table 1 show
PROPORTION SELF POLLINATION
that the main factor affecting the relation­
FIG. 2. Relationships between the proportion of
ship between self pollination and the fre­
pollination and the proportion of viable seeds for
quency of self seedlings is the viability of self
coastal Douglas-fir and noble fir. Number of em­
self embryos. For example, if n = 1 and bryos per ovule = one, two, and three. Proportion
the proportion of self pollination = 0. 5, of viable seeds when X = 0 (all cross pollination) set
the proportion of self seedlings is only at 1.0 (all seeds viable).
0.069 in Douglas-fir compared with 0.385
in noble fir. Beyond that, the families of
curves are similar with both polyem­
bryony and embryo competition favoring chance determining the winning embryos)
the outcross embryo augmenting the effect decreases the proportion of self seedlings
of low self-embryo viability. The aug- · from 0.069 to 0.054 (a reduction of 22 %)
menting effect occurs because of the in­ in Douglas-fir and from 0.385 to 0.365 (a
creasing concavity of the curves in Figure reduction of 5%) in noble fir. If the out­
1 and the increasing convexity of the cross embryo is always favored in pre-ger­
curves in Figure 2. In other words, poly­ mination competition, the decrease in the
embryony can both increase seed yield and proportion of self seedlings resulting when
decrease frequency of self seedlings.
n is increased from 1 to 2 is to 0.046 (a
Multiple embryos and embryo compe­ total reduction of 33 % from 0.069) in
tition have proportionately more effect Douglas-fir and to 0. 330 (a total reduction
when the viability of self embryos is low of 14% from 0.385) in noble fir.
than when it is high. To illustrate, if the
The potential effect of embryo compe­
proportion of self pollination is held at tition is greater when there are more via­
X = 0.5, increasing n from 1 to 2 (with ble embryos within the ovule. Again set-
~~
~\
'
"'
\,
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730
F . C. SORENSEN
TABLE 1. The proportions of self pollination and filled seeds associated with 2.5, 5.0, and 10.0% self
seedlings in the progeny of coastal Douglas-fir and noble fir trees. Median values have been used for the
embryo viability for each species. Five conditions of polyembryony and pregermination selection are com­
pared: one, two or three embryos per ovule (n = 1, 2, or 3) and, if viable self and cross embryos occur in the
same ovule, the embryo which ends up filling the embryo cavity can be the result of chance (chance) or the
result of early embryonic competition and selection, in which case it has been assumed that the cross embryo
always wins (cross wins).
DouglasRfir
Proportion of
Polyembryony and
self seedlings
embryo selection
0.025
0.050
0.100
1
2
n
n
n
n
n
=
=
=
=
=
n
n
n
n
n
= 1
= 2, chance
= 2, cross wins
n
n
n
n
n
=
=
=
=
=
1
2,
2,
3,
3,
chance
cross wins
chance
cross wins
= 3, chance
= 3, cross wins
1
2,
2,
3,
3,
chance
cross wins
chance
cross wins
Noble fir
Proportion
of self
pollination
Proportion
Proportion
Proportion
of filled
seeds 1
of self
pollination
of filled
seeds2
0.25
0.30
0.35
0.33
0.40
0.57
0.59
0.56
0.60
0.54
0.04
0.04
0.05
0 .04
0.05
0.55
0.55
0.55
0.55
0.55
0.41
0.47
0.53
0.51
0.56
0.46
0.48
0 .44
0.47
0.44
0.08
0.08
0 . 10
0.08
0.10
0.55
0.55
0.54
0.55
0.54
0.60
0.66
0.69
0.68
0. 71
0.33
0.34
0.32
0.35
0.32
0.15
0.16
0 . 19
0.16
0.20
0.53
0.53
0.53
0.53
0.52
Proportion of filled seeds after cross pollination (no self po1len) was 0. 75 for Douglas-fir.
Proportion of filled seeds after cross pollination (no self pollen) was 0.56 for noble fir.
ting X = 0. 5, if n = 2, changing the
competitive situation from a chance out­
come to one in which the viable outcross
embryo always wins is, for Douglas-fir,
accompanied by a decrease in frequency
of self seedlings from 0.054 to 0.046 (15%
reduction). If n = 3, the decrease in fre­
quency of self seedlings is from 0.049 to
0.039 (20% reduction). The pattern is the
same if embryo viability is high.
The values of X for which polyem­
bryony would confer the greatest advan­
tage are somewhat speculative. I have as­
sumed that the critical factors are to
maintain seed yield and to reduce the pro­
portion of self-pollination seedlings. Two
ovule classes appear to be most important
in this regard. One is the class which con­
tains a viable outcross embryo to replace
aborting self embryos (class 3). If n = 2
and a, (1 - a), c, and (1 - c) have non­
zero values, this class of ovules is at a
maximum when 2P 2 P3 is a maximum, or
when X = 1 - X =;= 0.5. If n = 3, it oc­
curs when (3P.JJ 3 2 + 6P?iJ:1J4 + 3P2 2P3) is
at a maximum. For Douglas-fir, this
expression is at a maximum at X = 0.44,
for noble fir at X = 0.42.
The second important class would ap­
pear to be ovules that contain at least one
viable outcross and one viable self embryo
(class 2). This class offers an opportunity
to eliminate a viable self embryo with no
loss in seed yield. If n = 2 and non-zero
values are specified for a, (1 - a), c, and
(1 - c), this class is at a maximum when
2PJJ 3 is at a maximum, or when X = Y =
0.5. If n = 3, this class is at a maxi­
mum when (3P 1 2P3 + 6PtP?iJ3 + 3PtP32 +
6PJJ:1J 4) is at a maximum. For Douglas­
fir, the expression is at a maximum at X
0.52, for noble fir at X= 0.51.
DISCUSSION
The data and calculations strongly in­
dicate that, as suggested by Sarvas (1962,
p. 130), one important role of polyem­
bryony and embryo viability in the genetic
system of conifers is to control the rela-
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POLYEMBRYONY AND SELFING IN CONIFERS
tionship between natural self pollination
(during wind pollination) and its inbreed­
ing consequences. In this role, the calcu­
lations further indicate that, although
polyembryony would be important over a
wide range of self pollination frequencies,
the maximum benefit would occur if nat­
ural self pollination made up about 50%
of the total pollination.
Some information about the proportion
of natural self pollination in conifers is
available. German workers (Fendrik,
1967; Schmidt, 1970; Stern, 1972; and
Muller, 1976), using a technique which
involved the capture of marker pollen,
have reported an average of 45-50% self
pollination during wind pollination of
stand-grown Pinus sylvestris and Picea
abies trees with abundant pollen produc­
tion. Sarvas (1962), using a more indirect
method, estimated 22-3 7% natural self
pollination in P . sylvestris in stands in
Finland. Koski (1970), using labelled pol­
len, reported 7% and 18% self pollination
in two trees of the same species in Fin­
land. The present author (unpubl.) has
made a preliminary estimate of an average
of about 50-60% natural self pollination
in six coastal Douglas-fir trees growing in
stands with openings and having abun­
dant pollen production. The proportion of
mutant seedlings after wind pollination
was equated to mutant-seedling propor­
tion after pollination with self-cross pollen
mixes using trees with known recessive
markers. While these data are meager,
they do suggest that the mating system or
natural pollen distribution system, at least
for species with relatively low self-embryo
viability, is adapted to take advantage of
polyembryony and embryo competition.
The reproductive pattern of coastal
Douglas-fir includes aspects suggesting
that low self-embryo viability and poly­
embryony have adaptive significance:
1) micro- and megasporangiate strobili in­
termingle in the crown;
2) release of pollen and receptivity of me­
gasporangiate strobili occur contem­
poraneously on the same tree;
3) individual megasj:>Orangiate strobili are
731
adapted to collecting pollen for several
days and then engulfing considerable
pollen (Dr. John N. Owens, Univ. Vic­
toria, British Columbia, pers. comm.);
4) Absence of mechanical barriers to self
pollination, self fertilization, and early
embryo development (Orr-Ewing,
195 7).
Coastal Douglas-fir often re-establishes it­
from pockets of seed trees following
widespread natural catastrophes, partic­
ular~y fires . Under these conditions, pro­
portiOns of self pollination could reach
0. 7, 0.8, or higher. This wide ranging
species is both highly heterozygous
(Campbell, 1972) and highly adapted to
local environmental conditions (Camp­
bell, 1979). Local adaptation would be
aided by restricted distribution of a rela­
tively large proportion of the effective pol­
lination, but this would also favor in­
breeding . Low self-embryo viability,
polyembryony, and pre germination selec­
tion would be important in reducing this
genetic load before it reached the seedling
population.
The situation would be different for no­
ble fir because of high viability of the self
embryos. Although polyembryony would
provide the opportunity for embryo selec­
tion, a change in the proportion of self
pollination would still be accompanied by
a nearly equivalent change in the propor­
tion of self seedlings. Since noble fir shows
considerable inbreeding depression in
growth, other mechanisms may be present
which inhibit self pollination or reduce self
fertilization . Spatial separation of micro-and megasporangiate strobili may be ef­
fective, or there may be other contributing
factors. Abies spp. also differ from Doug­
las-fir in the size and buoyancy of the pol­
len grain (smaller and lighter in Abies) and
in the pollination mechanism (simpler and
takes in less pollen in Abies) (Dr. John N.
Owens, Univ. Victoria, British Columbia,
pers. comm.). Additionally, long pollen
tubes are formed in Abies (Owens, pers.
comm.), but not in Douglas-fir (Allen and
Owens, 1972, p. 95), and cleavage polyse~f
This content downloaded from 166.6.105.57 on Tue, 20 Jan 2015 16:37:07 PM
All use subject to JSTOR Terms and Conditions
732
F. C. SORENSEN
embryony may occur in Abies (Owens and
Molder, 1977).
- - - . 1979. Genecology of Douglas-fir in a wa­
tershed in the Oregon Cascades. Ecology
60:1036-1050.
CHAMBERLAIN, C. ] . 1966. Gymnosperms: Struc­
SUMMARY
ture and Evolution. Dover Pub!., N.Y. 484 p.
Conifers are wind pollinated with no DoGRA, P. D. 1967. Seed sterility and disturbances
in embryogeny in conifers with particular refer­
reported restrictions on self pollination
ence to seed testing and tree breeding in Pina­
and self fertilization. Nevertheless, most
ceae. Stud. For. Suec., Nr. 45. 96 p.
species show large inbreeding depression FENDRIK, I. 1967. Entwicklung einer Indikator­
in growth; and the frequency of self seed­
Aktivierungsmethode zum Studium des Pollen­
fluges von Waldbiiumen. Diss., Tech. Univ.,
lings in wind-pollination progeny is gen­
Hannover.
erally low. In this paper the influence of
FOWLER, D. P. 1964. Pregermination selection
embryo viability, polyembryony, and pre­
against a deleterious mutant in red pine. For. Sci.
germination embryo selection on the re­
10:335-336.
lationship between natural self pollination FRANKLIN, E. C. 1968. Artificial self-pollination
and natural inbreeding in Pinus taeda. Ph.D.
and proportion of self seedlings is inves­
North Carolina St. Univ., Raleigh.
tigated. Species with low (coastal Doug­ - -Thesis,
- . 1969. Inbreeding depression in metrical
las-fir) and high (noble fir) self-embryo vi­
traits of loblolly pine (Pinus taeda L.) as a result
abilities are used as examples.
of self-pollination. Tech. Rep. No. 40, Sch. For.
Resour., North Carolina St. Univ., Raleigh. 19 p.
In Douglas-fir, the main factor reducing
the effects of self pollination is low via­ HAGMAN, M. 1972. On some factors influencing the
yield from seed orchards of Pinus sylvestris L.
bility of self embryos. Polyembryony and
and their interclonal and intraclonal variation.
the potential for pregermination selection
Forest Tree Improvement, Symposium on Seed
augment the effect of low self-embryo vi­
Orchards in Honour of C. Syrach-Larsen, p. 6783 . Akademisk Forlag, Kf/lbenhavn.
ability. Natural self pollination can reach
40-60% of total pollination without great­ HAGMAN, M ., AND L . MIKKOLA. 1963. Observa­
tions on cross-, self-, and inter-specific pollina­
ly increasing the proportion of self seed­
tions in Pinus peuce Griseb. Silvae Genet. 12:73lings in the seedling population.
79.
In noble fir, polyembryony allows for KOSKI, V. 1970. A study of pollen dispersal as a
mechanism of gene flow in conifers. Commun.
embryo selection; but frequency of self
For. Fenn. 70.4. 78 p.
seedlings increases nearly proportionately - -Inst.
-. 1971. Embryonic lethals of Picea abies and
to any increase in natural self pollination.
Pinus sylvestris. Commun. Inst. For. Fenn.
Other aspects of the mating system may
75.3. 30 p.
limit natural self pollination or self fertil­ KOSKI, V., AND E. MALMIVAARA. 1974. The role
of self-fertilization in a marginal population of
ization in this species.
Picea abies and Pinus sylvestris. Proc. Joint
IUFRO Meeting, S.02.04.1-3, Stockholm, ses­
ACKNOWLEDGMENTS
sion III. P. 155-165.
The manuscript has received several LINDGREN, D. 1975 . The relationship between self­
fertilization, empty seeds and seeds originating
thoughtful reviews. Particularly, I thank
from selfing as a consequence of polyembryony.
H.-R. Gregorious and W. T . Adams for
Stud. For. Suec., Nr. 126. 24 p.
helpful comments on the assumptions and · MERGEN, F., ] . BURLEY, AND G. M. FURNIVAL.
mathematics, ]. N . Owens for comments
1965. Embryo and seedling development in Pic­
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on the pollination mechanisms of coastal
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Douglas-fir and Abies, and W. ]. Libby
MULLER, G. 1976. Einschiitzung genetischer Ver­
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wandtschafts- und Inzuchtverhiiltnisse anhand
der Pollen- und Samenverbreitung bei Fichte
(Picea abies (L.) Karst.) und Kiefer (Pinus sil­
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POLYEMBRYONY AND SELFING IN CONIFERS
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733
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Corresponding Editor: R. Ornduff
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