Journal of Experimental Botany, Vol. 56, No. 419, pp. 2401–2407, September 2005
doi:10.1093/jxb/eri232 Advance Access publication 12 July, 2005
This paper is available online free of all access charges (see http://jxb.oupjournals.org/open_access.html for further details)
RESEARCH PAPER
Some physical properties of teosinte (Zea mays
subsp. parviglumis) pollen
Donald E. Aylor1,*, Baltazar M. Baltazar2 and John B. Schoper3
1
Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, New Haven,
CT 06504, USA
2
Pioneer Hi-Bred International-Mexico, Camino Viejo a Valle de Banderas, Km. 3, No. 19, Tapachula,
Nayarit, México
3
Pioneer Sementes Ltda. Unidade de Brası´lia, Rodovia DF 250, KM 20, Núcleo Rural Santos Dumont,
Lote 50 Caixa Postal 8283, Planaltina, Brasilia, DF, Brazil
Received 15 February 2005; Accepted 24 May 2005
Abstract
In parts of the world where teosinte and maize are
grown in close proximity, there is concern about gene
flow between them. Pollen is the primary vehicle for
gene flow. Quantifying the biophysical properties of
pollen, such as its settling speed and dehydration rate,
is important for evaluating outcrossing potential.
These properties were measured for teosinte (Zea
mays subsp. parviglumis) pollen. Pollen was found to
have an average settling speed of 0.165 m s21, which
agrees well with theoretical values based on the size of
the pollen grains. The conductance of the pollen wall
for water was derived from the time rate of change of
pollen grain size and gave an average conductance of
3.4231024 m s21. Water potential, w, of teosinte pollen
was determined at various values of relative water
content (dry-weight basis), h, by using a thermocouple
psychrometer and by allowing samples of pollen to
come to vapour equilibrium with various saturated salt
solutions. Non-linear regression analysis of the data
yielded w (MPa) 5 24.13 h21.23 (r 250.77). Results for
conductance and w were incorporated into a model
equation for the rate of water loss from pollen grains,
which yielded results that agreed well (r 250.96) with
observations of water loss from pollen grains in air.
The data reported here are important building blocks in
a model of teosinte pollen movement and should
be helpful in establishing the main factors influencing
the degree and the direction of pollination between
teosinte populations and between maize and teosinte.
Key words: Aerial dispersal, genetically modified crops, geneflow models, pollen viability, settling speed, survival, vapour
pressure deficit, water content, water potential, Zea mays.
Introduction
Teosinte (Zea spp.) is an annual and perennial grass
endemic to Mexico and Central America. Current theories
place teosinte at the centre in the development of and in the
variation found among the principal landraces in Mexico
(Wellhausen et al., 1952; Mangelsdorf, 1974; Matsuoka
et al., 2002). As such, the potential use of teosinte in maize
breeding has been evaluated since the 1950s. Researchers
concluded that teosinte is a valuable germplasm for maize
improvement, providing resistance to diseases and other
abiotic stresses (Reeves, 1950), as well as quantitative traits
(Cohen and Galinat, 1984; Casas et al., 2001). In addition,
teosinte is itself culturally important because of the beverages and special culinary dishes that are prepared,
particularly in regions of Mexico where it grows in the
wild.
Teosinte is represented by annual diploid species
(Zea mays ssp. mexicana and Zea mays ssp. parviglumis)
and by perennial diploid (Zea diploperennis) and tetraploid
(Zea perennis) species. It is found within tropical and
subtropical areas of Mexico in isolated populations of
varying sizes, sometimes occupying several square kilometres but most often in patches of less than a square
kilometre (Sánchez et al., 1998). Natural gene exchange
between maize and teosinte (Wilkes, 1977), as well as
* To whom correspondence should be addressed. Fax: +1 203 974 8502. E-mail: [email protected]
ª The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
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2402 Aylor et al.
hybridization and human selection, have led to the genetic
diversity that is found in Mexican farmers’ fields today. As
a result, there are at least 60 maize landraces in Mexico
(Sánchez et al., 2000).
Maize-to-maize gene flow is well documented (Baltazar
and Schoper, 2002). Mexican farmers have promoted gene
flow by planting side-by-side landraces, improved openpollinated species, and hybrids, thereby increasing the
genetic diversity in Mexico (Bellon and Brush, 1994;
Bellon and Risopoulos, 2001). Low levels of gene flow
between maize and teosinte have been documented
(Baltazar et al., 2005). Results of this study support the
hypothesis that gene flow and subsequent introgression of
maize alleles into teosinte populations most likely result
from crosses where teosinte first pollinates maize. This
underscores the importance of understanding teosinte
pollen movement. The resultant hybrids then backcross
with teosinte to introgress the maize alleles into the teosinte
genome. The initial pollination from teosinte onto maize
silks most likely represents the rate-limiting step in the
eventual introgression of maize alleles into the teosinte
genome and may help explain why teosinte continues to
coexist as a separate entity, even though it normally grows
in the vicinity of much larger populations of maize.
Studies of isolation distance between maize plants in
Tapachula, Nayarit, showed that very limited cross-pollination occurred at 100 m and no pollination occurred at
distances beyond 200 m (Luna et al., 2001). Wind and
atmospheric conditions at the time pollen was released into
the atmosphere were critical factors affecting pollen movement and longevity (Luna et al., 2001; Baltazar and
Schoper, 2001; Baltazar et al., 2005). Other studies showed
that pollination could be as high as 60% between contiguous rows, but pollination from more remote rows decreased with increasing distance such that no outcrosses
were found 32 m away from the pollen source (Castillo and
Goodman, 1997; Cervantes, 1998). However, wind speed
and direction, turbulence, and pollen settling speed affect
pollen movement (Di-Giovanni and Kevan, 1991; Aylor
et al., 2003), and variability in these factors can theoretically change the distance of cross-pollination.
At present, little is known about teosinte pollen movement in the atmosphere. To help fill this gap, data on the
settling speed, water potential, germination rate, drying
rate, and the resistance to water loss of teosinte pollen
are presented in this paper. These data on the physical
properties of teosinte pollen are important building blocks
in a model of teosinte pollen movement and, as such, will
be helpful in establishing the main factors influencing the
degree and direction of pollination between teosinte populations and between maize and teosinte. Consequently,
they will help to increase our understanding of the geneflow process in Mexico and Central America where teosinte
and maize populations grow in geographic proximity and
flower synchronously.
Materials and methods
Source of pollen
The pollen used in all tests was freshly collected from teosinte plants
(Zea mays subsp. parviglumis, Accession: Ames 21785, Lot:
92ncao01, North Central Plant Introduction Station, Ames, Iowa,
USA) grown in the greenhouse in 4.0 l plastic pots in a soil mixture of
Promix BX, sand, and Perlite in the ratio (2:1:1 by vol.). New
plantings were begun in the greenhouse every four to five days to
ensure a steady and continuous supply of fresh pollen over several
weeks. Fresh pollen was collected from anthers on plants by gently
tapping the stems of plants while holding a collecting tray below the
tassels. To help ensure that freshly produced pollen was collected, the
tassels were ‘milked’ of pollen, either at the end of the previous day
or in the morning of the same day, to remove old pollen prior to
a collection of fresh pollen used in experiments. Any anthers
inadvertently collected with the pollen were removed from the dishes
prior to exposure using forceps.
Settling speed
Measurements of the settling speed of teosinte pollen in still air, vS,
were made by timing the rate of fall of pollen grains inside a glass
settling tube (Aylor, 2002). In particular, the time for pollen to fall
a distance of 1.0 m was determined using a stopwatch. In addition,
a microscope slide was momentarily placed just below the opening at
the bottom of the tube to capture (by sedimentation) the particular
group of pollen grains being timed. These grains were immediately
examined microscopically to determine their characteristic dimensions (principal diameters) and shapes. In this way, individual
collections of pollen were used to measure the settling speed and
size of pollen grains over a range of pollen sizes.
Teosinte pollen grains are shaped like prolate spheroids, where L1
and L 2 are major and minor principal diameters of the
ppollen
ffiffiffiffiffiffiffiffiffiffi grain. A
volume-equivalent diameter, De, given by: De = 3 L1 L22 was determined. The ratio L1/L 2 for the teosinte pollen tested here was
1.1660.06. For this small degree of eccentricity, it is reasonable to
take the dynamical shape factor equal to 1.0 and to calculate settling
speed based on a volume-equivalent sphere (Fuchs, 1964; Leith,
1987).
The variation of settling speed with pollen size can be expressed
theoretically by (Fuchs, 1964; Aylor, 2002):
v2s =
4qP gDe
3qa cd ðvS Þ
ð1aÞ
where
cd = 24Re1 ð1 + 0:158Re2=3 Þ
ð1bÞ
and where qa and qP are the density of the air and of the pollen grain,
respectively, cd is a drag coefficient which varies with vS, Re is the
Reynolds number based on the characteristic diameter of the pollen
grain, and g is gravity. For the calculations presented in this paper, qP
has been taken to be a constant equal to 1150 kg m3. For particles
small enough to obey Stokes law (Fuchs, 1964), vS can be calculated
using equation 1a and the first term (only) of equation 1b.
Water potential versus water content
The relative water content of teosinte pollen is expressed as a fraction
of dry weight, namely, h=mw/md, where mw is the mass of water in
a sample of teosinte pollen and md is the mass of the same sample of
oven-dry pollen. The value of mw at a given time was determined by
subtracting md from the pollen mass at that time. For the teosinte
pollen used in these studies, h ranged from ;1.35 for hydrated
teosinte pollen, fresh from the anther, to zero for oven-dry pollen.
Properties of teosinte pollen
Germination versus water content
Teosinte pollen was placed in individual sample weighing dishes and
allowed to dehydrate slowly. Periodically, subsamples of pollen were
taken and placed in liquid germination medium, and allowed to
germinate. The liquid medium used for germination was essentially
that described by Walden (1994), except that the water potential of
the medium was adjusted by adding a larger amount (222 g l1) of
sucrose (Aylor, 2003). The pollen remaining in the sample dishes
after the subsamples for germination were taken was weighed to
obtain a ‘fresh weight’ and then they were placed in a drying oven
and weighed again to determine their dry weight (Aylor, 2003).
A pollen grain was counted as germinated if the length of the germ
tube was > one diameter of the pollen grain (i.e. approximately
75 lm long).
Drying rates
The drying rate of teosinte pollen was determined by two methods
(Aylor, 2003): (i) by repeated reweighing of a small amount of pollen
(in a thin layer averaging 1–2 grains thick) on the bottom of an
aluminium weighing dish that was held over saturated salt solutions
between each weighing and (ii) by measuring the principal diameters
of individual grains using a microscope (at 3100) at various times
during their exposure to air-streams having a range of temperatures
and humidity. The second method utilized a small, specially
constructed, flow chamber attached to the stage of the microscope
(for details see Aylor, 2003). The principal diameters of the grains
were measured microscopically at frequent intervals and were
recorded. Changes in these diameters were converted to changes in
mass of individual pollen grains by converting them to volume
changes (using the volume–equivalent diameter, De) and assuming
that volume changes were directly a result of water loss. The change
in size and shape of five to eight pollen grains was observed for each
determination at a particular vapour-pressure deficit (VPD). The
experiment was repeated 11 times for VPDs ranging from 0.6 to
1.9 kPa. These data were used to derive pollen ‘wall’ conductance
values, described in the next section.
Calculated pollen wall conductance
The driving force for water loss from a pollen grain is the vapour
pressure difference between an evaporating surface within the pollen
wall and the ambient air outside the grain. Following the arguments in
Aylor (2003), we have conceptually combined the conductance of the
plasma membrane, the inner and outer wall layers, and the aperture
area into a single conductance, which is referred to in this paper as the
conductance of the pollen wall.
The instantaneous rate of loss of relative water content, dh/dt (s1),
from a pollen grain can be expressed as (Aylor, 2003):
dh gv ðhÞAp ðhÞMw pVsat ðTa Þ
RH
hr ðhÞ =
ð2aÞ
dt
md RTa
100
and
ð2bÞ
where h (=mw/md) is the relative water content, md is the mass of dry
pollen, gv (m s1) is the overall conductance for water loss of the
pollen wall and of the surrounding boundary layer of air, A p (m2)
is the surface area of the pollen grain, Mw (0.018 kg mol1) is the
molecular weight of water, R (8.3143 J mol1 8K1) is the universal
gas constant, T (8K) is temperature, hr(h) is the ‘effective’ relative
humidity inside the wall of the pollen grain, RH is the relative
humidity (%) of the surrounding air, w (MPa) is the net water
potential at the evaporating surface, and Vw (1.83105 m3 mol1) is
the molar volume of water (Campbell and Norman, 1998; Aylor,
2003). Equation 2 is non-linear and was solved numerically using
a fourth order Runge–Kutta scheme, subject to the initial condition
h(t=0)=h0.
It can be shown (Aylor, 2003) that the conductance of the pollen
wall is much less than the conductance of the surrounding boundary
layer, so that gv in Equation 2a can be replaced with gw, where gw is
the conductance of the wall. An estimate was obtained for the
conductance of the pollen wall following the method described in
Aylor (2003). It is reasonable to assume that gv(’gw), Ap, and hr are
all approximately constant (i.e. do not depend on h) over a limited
range of h near full hydration. In this case, Equation 2 indicates that h
decreases approximately linearly with time for a limited range of
water content, near full hydration of the pollen grain. Estimated
values of gv were obtained from Equation 2 by expressing the change
in mass of water in the grain in finite difference form, i.e. as md Dh/Dt,
where the change in mass was obtained from the change in size of
individual grains.
Results
Size and settling speed
The mean volume-equivalent diameter (De) of the teosinte pollen used in these experiments was 7263 lm
(median=71 lm, range 63–80 lm). The average settling
speed, vS, was 0.165 (60.018) m s1 (median=0.163, range
0.131–0.216 m s1) (Fig. 1).
0.20
0.18
vS (m s-1)
A relationship between water potential, w (MPa), and relative
water content, h (dimensionless), for teosinte pollen was obtained
using two methods (Aylor, 2003). In the first method, pollen was first
allowed to reach moisture equilibrium above various saturated salt
solutions having relative humidity ranging from ;11% to 94% and
then the equilibrium water content of the pollen was determined
gravimetrically. In the second method, pollen water potential was
determined using a thermocouple psychrometer (comprised of
a Model HR-33T Dew point microvolt meter and a Model C-52
sample chamber, Wescor, Inc., Logan, Utah, USA) and water content
was again determined gravimetrically.
2403
wVw
aVw
hr = exp
= exp
RT
RThb
0.16
0.14
0.12
0.10
60
65
70
75
80
De (µm)
Fig. 1. Settling speed in still air, vS (m s1), versus volume-equivalent
diameter, De (lm), for teosinte pollen. Shown are the measured values
(solid circles) and the theoretical curve given by Equation 1 (solid line;
r2=0.993, n=86). The well-known Stokes law result (dashed line), shown
for comparison, gives values that are about 10% too high.
2404 Aylor et al.
100
Germination (%)
ψ (MPa)
-20
-120
-220
-320
0.0
A
0.4
0.8
1.2
1.6
10
1
0.1
B
0.01
0.0
0.5
Relative water content, θ
1.0
1.5
2.0
Relative water content, θ
Fig. 2. (A) Water potential, w (MPa), versus relative water content, h (dimensionless), for teosinte pollen. Measured values are shown by solid circles
and open diamonds. The fitted line is given by w= 4.13 h1.225 (r2=0.77, df=63). (B) Germination, G (%), versus h for teosinte pollen (solid circles).
The fitted line is a non-linear fit of the data to a Weibull distribution given by Gð%Þ = 62½1expðððh0:05Þ=0:23Þ1:7 Þ(r2=0.92, df=53).
Conductance of pollen wall
Values for the wall conductance, gw, of teosinte pollen were
determined by measuring changes in size of individual
grains. The mean value of gw (Fig. 3) was 3.4260.13
(sem)3104 m s1 (2.913104 median, range 0.2–
13.93104 m s1, n=297), determined by measuring the
change in sizes of individual grains exposed to a range of
vapour pressure deficits from 0.7 kPa to 1.9 kPa.
Drying rate of pollen
The change in relative water content, h, of teosinte pollen
was strongly dependent on RH. When exposure was expressed as a function of VPD3time (Fig. 4A), then most
data fell onto a single curve, confirming the physical
dependence on VPD, given by Equation 2. The model
results followed closely the shape of the pollen drying
curves (Fig. 4A) and explained about 96% of the variation
in the data (Fig. 4B). There was no overall trend in the
variances over the entire range of predicted h, however,
there was a tendency for the variances to be larger near midrange values than at either end of the range. Differences
between predicted and observed values, tested using a non-
1
0.8
CDF
Water potential and germination percentage
versus RWC
The water potential, w (MPa), of teosinte pollen decreases
relatively slowly with changes in relative water content, h
(dimensionless), for values of h >0.5, and it decreases more
and more rapidly as h approaches zero (Fig. 2A). The
relationship between w and h can be expressed by an
equation of the form w= 4.13 h1.225 (r2=0.77, df=63).
Pollen germination percentage remained high for values
of h >0.3, but decreased rapidly for lower values of h (Fig.
2B). The data over the entire range of h were described
well by a cumulative Weibull distribution given by
G = 62½1expðððh0:05Þ=0:23Þ1:7 Þ determined by nonlinear regression (r2=0.92, df=53). This equation gives
G/Gmax >0.9 for h >0.43, and G/Gmax=0.5, 0.25, and 0 at
h=0.24, 0.16, and 0.05, respectively.
0.6
0.4
0.2
0
0.0000
0.0005
0.0010
gw(m
0.0015
s-1)
Fig. 3. Cumulative distribution function (CDF) of pollen wall conductance values, gw (m s1), for teosinte pollen (solid line). The mean, median, and standard deviation of gw are 3.42, 2.91, and 2.323104 m s1
(n=297), respectively. The Gaussian CDF (dashed line) for the same
mean and standard deviation is shown for comparison.
parametric Kolmogorov–Smirnov test, were not significantly different (P=0.11).
The model was used to compare relative drying rates
for maize and teosinte pollen. The model indicates that teosinte pollen dries about 40% faster than hybrid maize pollen
(Fig. 5). The difference in the drying rates for teosinte and
maize pollen largely reflects the difference in the grouping
gwAP/md in the model (Equation 2) for these two kinds of
pollen. For these calculations, the wall conductance, gw,
was set equal to the median values, i.e. 2.913104 m s1
for teosinte and 2.193104 m s1 for maize pollen. The
curves level out as they approach the equilibrium values of
water content, which are obtained by taking the inverse of
Equation 2b and then using the parameter values given in
this section. Equilibrium water contents of teosinte pollen
(at 23.5 8C) for RH=20, 33, 54, and 75% were about 5.3,
6.9, 10.7, and 18.8%, respectively.
Discussion
Pollen movement studies in the environment in the genus
Zea have been conducted with maize pollen (Luna et al.,
Properties of teosinte pollen
1.0
0.8
0.6
0.4
1.0
B
0.8
0.6
0.4
0.2
0.2
0.0
1.2
A
1.2
Predicted
Relative water content,
1.4
2405
0
50
100
150
200
VPD x t (kPa x min)
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Observed
Fig. 4. (A) Decrease in the relative water content, h, of teosinte pollen plotted as a function of vapour pressure deficit (VPD)3time. Symbols represent
successive values of h for pollen exposed to air at a temperature of 22.5 8C and RH values of 28% (squares), 33% (circles), and 54% (diamonds). The
solid line is calculated using Equation 2 for RH=54%. (B) Predicted versus observed values of h during the time-course of teosinte pollen drying in
various T and RH conditions were highly correlated (r=0.98, n=180, P <107). The solid line is the 1:1 line.
1.4
Relative water content, θ
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
Time (min)
Fig. 5. Relative water content, h, versus time (calculated by Equation 2)
for teosinte pollen (solid line) and for corn pollen (dashed line) exposed
to a VPD=1.3 kPa (T=23.1 8C, RH=54%). The parameter values used in
the calculation for corn pollen were for a commercial hybrid and were
taken from Aylor (2003).
2001). However, very little is known about teosinte pollen
mobility in the atmosphere. As far as we are aware, this
study is the first to determine physical characteristics of
teosinte pollen, which can contribute to defining the main
parameters involved in gene flow in the genus Zea as
a whole.
It is well known that pollen viability in the genus Zea
is sensitive to water loss (Barnabas, 1985; Buitink et al.,
1996; Luna et al., 2001); however, few quantitative measures for evaluating this sensitivity are available. Water loss
from pollen is a complex process and involves water
movement across a membrane and two wall layers. It is
difficult to separate the effects on water movement of these
various layers in intact pollen. At present, it is not possible
to quantify the wall conductance in terms of its component
parts, although recent experiments suggest that the exine
may not be the main factor in limiting water loss, at least
from pine pollen (Bohne et al., 2003). In this paper, the
effects of these various layers were combined into an
overall wall conductance and then it was used to quantify
the rate of water loss from teosinte pollen. The distribution
of wall conductance found was skewed toward higher
values (Fig. 3), resulting in the mean being considerably
higher than the median. It was assumed that the median
pollen wall conductance (;2.913104 m s1) is most
representative of the population and it was then used to
evaluate the rate of water loss from teosinte pollen grains.
However, it is important to remember that using a single
number for the wall conductance represents only part of the
story and that models for gene flow in the environment will
also need to account for the variation in conductance among
members of the population of pollen grains (Fig. 3).
In this study, germination was used as a measure of
pollen viability. This measure may tend to overestimate
viability because not all of the pollen grains that germinate
will necessarily be able to effect fertilization. Nevertheless,
the ability of the pollen to recover and germinate in our
in vitro test after drying to a particular water content should
give a useful indication of how drying might ultimately
effect potential fertilization.
Previous studies have focused on the biology of teosinte
pollen (Baltazar and Schoper, 2001). Studies in Mexico
have shown that teosinte pollen is about 30% smaller than
maize pollen (Baltazar and Schoper, 2001; Baltazar et al.,
2005). The smaller size of teosinte pollen suggests that it
could travel longer distances than maize pollen from the
pollen source, increasing the possibility of outcrossing at
longer distances. These studies have also shown that
teosinte pollen is 30–50% more susceptible to desiccation
after being exposed to environmental conditions, which
would tend to lessen the likelihood of outcrossing.
In this study, it was found that teosinte pollen settles at
a rate that is about 40% slower than found for hybrid maize
pollen (Di-Giovanni et al., 1995; Aylor, 2002) and dries
about 40% faster than maize pollen (Table 1). This faster
drying rate should result in about a 40% shorter survival
time for teosinte pollen compared with hybrid maize pollen
(Fig. 2B; Aylor, 2004). In view of these differences in
2406 Aylor et al.
Table 1. Comparison of mean values of volume equivalent
diameter (De), settling speed (vS), and pollen wall conductance
to water vapour (gw) for teosinte (Zea mays subsp. parviglumis)
and maize (Zea mays L.) pollen
The coefficients in the expressions for water potential (w) versus relative
water content (h), w = a1 hb1 , and germination percentage (G) versus
relative water content, G=c1 ð1expðððh0:05Þ=d 1 Þe1 ÞÞ for both kinds
of pollen are also given.
Property (units)
Teosinte
Maizea
De (lm)
vS (m s1)
gw (m s1)
Mean
Median
w (MPa)
Parameter a1
Parameter b1
G (%)
Parameter c1
Parameter d1
Parameter e1
72
0.17
91b
0.26b
3.423104
2.913104
2.653104c
2.193104c
4.13
1.23
3.22c
1.35c
62
0.23
1.70
65c
0.29c
1.92c
a
Hybrid maize (variety 37M81, Pioneer Hi-Bred International,
Johnston, IA).
b
Data from Aylor (2002).
c
Data from Aylor (2003).
physical properties of teosinte and maize pollen several
factors related to their dispersal can be considered. Because
of the smaller settling speed of teosinte pollen it is expected
that it would move farther than maize pollen from the
pollen source before being deposited. On a per plant basis,
teosinte produces more pollen than landraces and commercial hybrid maize (Baltazar et al., 2005), which also
contributes to its ability to pollinate at a distance. The effectiveness of the more prolific and farther-moving pollen
might be partially or completely offset by the shorter pollen longevity of teosinte in the atmosphere. The net effect
of these dynamics will depend on several environmental
factors such as wind speed and turbulence and the vapour
pressure deficit of the air. The model equations for settling
speed and dehydration rate presented in Equations 1 and 2,
respectively, offer a means for evaluating dispersal potential for a range of environmental conditions.
The genetic exchange between maize and teosinte is
dependent on many factors, of which pollen movement is
but one. Other factors include (i) the spatial isolation of the
two species, (ii) the differences in flowering time of the two
species, and (iii) the fitness of the hybrids combined with
the types of selection operating in the teosinte populations.
Wilkes (1977) and Sánchez et al. (1998) made a thorough
review of teosinte distribution and characterization in
Mexico. They described dates of planting across the
Mexican Republic, and determined the probability of outcrossing with maize due to synchronization of flowering.
Typically, the growing season for teosinte in Mexico is
June through November. Seeds germinate with the beginning of the summer rains and growth parallels but plant
development in teosinte tends to be somewhat later than for
the local cultivated maize. Nonetheless, teosinte sheds
pollen for about a month, and the earliest shedding teosinte
plants tend to overlap flowering in maize at the end of their
silking period, which allows for the possibility of maize
forming fertile hybrids between maize and teosinte.
The physical properties of teosinte pollen determined in
this study are important components of a model for pollen
transport in the atmosphere and will aid in evaluating one of
the main factors influencing gene flow in the genus Zea in
Mexico and Central America. Likewise, it will contribute to
our basic knowledge, required as part of any estimation of
the potential impact of introducing transgenic maize into
maize’s centre of origin.
Acknowledgements
We thank P Thiel for excellent technical assistance, RJ Salvador
for supplying the teosinte seed, and N Schultes and M Boehm
for reviewing the manuscript. This material is based upon work
supported in part by Hatch Funds and by the Cooperative State
Research, Education, and Extension Service, U. S. Department of
Agriculture, under Agreement No. 2004–33522–15044.
References
Aylor DE. 2002. Settling speed of corn (Zea mays) pollen. Journal of
Aerosol Science 33, 1599–1605.
Aylor DE. 2003. Rate of dehydration of corn (Zea mays L.) pollen in
the air. Journal of Experimental Botany 54, 2307–2312.
Aylor DE. 2004. Survival of maize (Zea mays) pollen exposed in the
atmosphere. Agricultural and Forest Meteorology 123, 125–133.
Aylor DE, Schultes NP, Shields EJ. 2003. An aerobiological
framework for assessing cross-pollination in maize. Agricultural
and Forest Meteorology 119, 111–129.
Baltazar BM, Sánchez GJJ, de la Cruz-Larios L, Schoper JB.
2005. Pollination between maize and teosinte: an important determinant of gene flow in Mexico. Theoretical and Applied
Genetics 110, 519–526.
Baltazar BM, Schoper JB. 2001. Maize pollen biology, pollen drift
and transgenes. Proceedings of the 56th corn and sorghum seed
research conference, 5–7 December, 2001, Chicago, IL., 156–170.
Baltazar BM, Schoper JB. 2002. Crop-to-crop gene flow: dispersal
of transgenes in maize during field tests and commercialization.
Proceedings of the 7th international symposium on the biosafety
of genetically modified organisms, Beijing, China, 10–16 October,
2002, 24–33.
Barnabas B. 1985. Effect of water loss on germination ability of
maize (Zea mays L.) pollen. Annals of Botany 55, 201–204.
Bellon MR, Brush SB. 1994. Keepers of maize in Chiapas, Mexico.
Economic Botany 48, 196–209.
Bellon MR, Risopoulos J. 2001. Small-scale farmers expand the
benefits of improved maize germplasm: a case study from Chiapas,
Mexico. World Development 29, 799–811.
Bohne G, Richter E, Woehlecke H, Ehwald R. 2003. Diffusion
barriers of tripartite sporopollenin microcapsules prepared from
pine pollen. Annals of Botany 92, 289–297.
Buitink J, Walters-Vertucci C, Hoekstra FA, Leprince O. 1996.
Calorimetric properties of dehydrating pollen: analysis of a
Properties of teosinte pollen
desiccation-tolerant and intolerant species. Plant Physiology 111,
235–242.
Campbell GS, Norman JM. 1998. An introduction to environmental biophysics. New York: Springer-Verlag.
Casas SJF, Sánchez GJJ, Ramı́rez DJL, Ron PyS, Montes HJ.
2001. Rendimiento y sus componentes en retrocruzas maı́zteocintle. Revista Fitotecnia Mexicana 24, 17–26.
Castillo GF, Goodman MM. 1997. Research on gene flow between
improved land races. In: Serratos JA, Willcox MC, CastilloGonzalez F, eds: Proceedings of a forum: gene flow among maize
landraces, improved maize varieties, and teosinte: implications
for transgenic maize, CIMMYT, Mexico, DF, 67–72.
Cervantes MJE. 1998. Infiltracion genetica entre variedades locales
e introducidas de maiz de sistema tradicional de Cuzalapa, Jalisco.
Tesis Doctoral. Colegio de Postgraduados, Montecillo-Texcoco,
Edo. de Mexico.
Cohen JI, Galinat WC. 1984. Potential use of alien germplasm for
maize improvement. Crop Science 24, 1011–1015.
Di-Giovanni F, Kevan PG. 1991. Factors affecting pollen dynamics
and its importance to pollen contamination: a review. Canadian
Journal of Forest Research 21, 1155–1170.
Di-Giovanni F, Kevan PG, Nasr ME. 1995. The variability in
settling velocities of some pollen and spores. Grana 34, 39–44.
Fuchs NA. 1964. The mechanics of aerosols. New York: Macmillan
Publishing Company.
Leith D. 1987. Drag on non-spherical objects. Aerosol Science and
Technology 6, 153–161.
2407
Luna VS, Figueroa JM, Baltazar BM, Gomez RL, Townsend R,
Schoper JB. 2001. Maize pollen longevity and distance isolation requirements for effective pollen control. Crop Science 41,
1551–1557.
Mangelsdorf PC. 1974. Corn. Its origin, evolution and improvement. Cambridge: The Belknap Press, Harvard University Press.
Matsuoka Y, Vigouroux Y, Goodman MM, Sánchez GJJ,
Buckler E, Doebley J. 2002. A single domestication for maize
shown by multilocus microsatellite genotyping. Proceedings of
the National Academy of Sciences, USA 99, 6080–6084.
Reeves RG. 1950. The use of teosinte in the improvement of corn
inbreds. Agronony Journal 42, 248–251.
Sánchez GJJ, Kato YTA, Aguilar MSM, Hernandez CJM, Lopez
CAR, Ruiz JAC. 1998. Distribución y caracterización del teocintle. Libro Te´cnico No. 2 del Instituto Nacional de Investigaciones
Forestales, Agrı´colas y Pecuarias (INIFAP).
Sánchez GJJ, Stuber CW, Goodman MM. 2000. Isozymatic diversity of the races of maize of the Americas. Maydica 45, 185–203.
Walden DB. 1994. In vitro pollen germination. In: Freeling M,
Walbot V, eds: The maize handbook. New York: Springer-Verlag,
733–734.
Wellhausen EJ, Roberts LM, Hernández EX (in collaboration
with Mangelsdorf PC). 1952. Races of maize in Mexico.
The Bussey Institution of Harvard University.
Wilkes HG. 1977. Hybridization of maize and teosinte in Mexico
and Guatemala and the improvement of maize. Economic Botany
31, 254–293.