Does Mating Cause Eggs to Mature? (Continued)
Abstract | Introduction
| Methods | Results | Discussion | Acknowledgments |
References | Karen's Research Questions
Results
Effect of mating on the presence of mature oocytes
Figure 1 summarizes the state of oocyte maturation in both virgin
and mated females. Egg maturation takes from two to three days; at age one day no
oocytes were visible in any females, by age three days half of the females had yolked
oocytes, and by age four some females had mature oocytes. We used a log-linear model
to test for independence of oocyte development state and mating treatment. For this
test, we combined ages four and five; six and seven; and eight, nine and ten to
make three age categories. Females dissected at ages one, two and three were not
included in the test, because of the small number of mated females dissected at
these ages. Given age, oocyte development state is independent of mating treatment
(Pearson X2 coefficient = 5.28, df = 9, p = 0.809).
Figure 1: State of oocyte development in female monarchs dissected at different
ages. Females were categorized according to their most mature oocytes. For example,
those with mature oocytes also contained yolked and unyolked oocytes, but they are
only counted as having mature oocytes.
Effect of mating on the number of mature oocytes
Figure 2 shows the number of mature oocytes as a function of
virgin female age. Females continue to produce mature oocytes over their first ten
days, even when they do not mate. A linear regression of number of mature oocytes
on female age and mass (Table 1a) indicates that oocyte number
increases with age. The effect of virgin female mass on oocyte number is not significant
at the 0.05 level of confidence. We tested for effects of female age, mass, and
the number of days that had elapsed since mating on mature oocyte number in mated
females. Time since mating was included in the model in two different ways; once
with one, two, and three days separated (Table 1b), and once
with one and two days combined (Table 1c). This was done because
visual (see Figure 3) and statistical (see below) analyses of
the data suggested that, for a given age, there is no difference in the number of
mature oocytes in mated females dissected one or two days after mating. Again, there
is a strong effect of female age on mature oocyte number. Initial female mass had
a small, but statistically significant, negative effect. Time since mating
had a positive effect in both models, but the adjusted R2 value for the
model that combined one and two days is higher.
Figure 2: Number of mature oocytes as a function of virgin female age. The purple
stars indicate means, which were calculated using both zero and positive values.
Symbols at zero include females with immature oocytes, and some represent more than
one female.
TABLE 1. EFFECTS OF AGEa, FEMALE MASS, AND TIME SINCE MATING ON NUMBER
OF MATURE OOCYTES
a. Virgin females
|
Predictor |
Coefficient |
Std. Error |
P |
Adj. R2b |
|
age |
6.71 |
1.75 |
0.0005 |
0.541 |
|
female mass |
-0.052 |
0.0256 |
0.0562 |
0.572 |
b. Mated females (1, 2, and 3 days after mating separate)
|
Predictor |
Coefficient |
Sth. Error |
P |
Adj. R2 |
|
age |
8.35 |
2.81 |
0.0053 |
0.640 |
|
female mass |
-0.10 |
0.0336 |
0.0040 |
0.690 |
|
tsmc |
17.5 |
6.76 |
0.0138 |
0.732 |
c. Mated females (1 and 2 days after mating combined)d
|
Predictor |
Coefficient |
Sth. error |
P |
Adj. R2 |
|
age |
8.31 |
2.36 |
0.0012 |
0.644 |
|
mass |
-0.14 |
0.032 |
0.0001 |
0.694 |
|
tsm (dummy) |
43.3 |
10.9 |
0.0003 |
0.782 |
aOnly females aged four days and older were included in
regressions, since no eggs are mature before this age.
bAdjusted R2 from stepwise analysis of variance
cTime Since Mating; number of days after mating that female was dissected
dTsm included as dummy variable, with 1 and 2 days combined
There is no difference between the number of mature oocytes in mated females dissected
one or two days after mating and in virgin females dissected at the same age (Figure 3; p > 0.15 for ages four to ten days, t-tests for
unequal sample sizes and variances). However, females dissected three days after
mating (at ages eight, nine, and ten days) had more mature oocytes than both virgin
females and mated females of the same age dissected one or two days after mating
(p < 0.02 for all ages; t-tests for ages eight and nine, sign test age
ten).
We observed no oviposition behavior on the part of mated females before they were
dissected, nor did we detect any eggs on either the sides of the cage or non-hostplants
(mostly grass) in the cage. While it is possible that some oviposition on inappropriate
substrates did occur, we have no evidence of this.
Figure 3: Number of mature oocytes as a function of mated female age. Purple stars
indicate means for virgin females of the same age for comparison. Other symbols
refer to the number of days after mating that females were dissected.
Timing of mating
Forty-two of the mating-treatment females (86%) had mated by age eight days, when
we stopped putting males in the mating cage. The percentage of females that mated
at each age is shown in Figure 4. Before age four, when no females
had mature oocytes, only 14% of the females had mated. By age six days, when all
virgin females had either yolked or mature oocytes, 70% of them had mated.
Figure 4: Absolute and cumulative percentages of females that mated as a function
of age
Discussion
Effects of mating on oocyte development
Experimental virgin females contained mature eggs (Figures 1
and 2), clearly indicating that mating is not required to stimulate
oogenesis in monarchs. This is contrary to Ehrlich and Ehrlich (1978) and Drummond
(1984) who stated that mating is required to induce oogenesis in monarchs. Our results
do not tell us what does stimulate oogenesis in monarch. The induction of oogenesis
in Lepidoptera is often caused by increased levels of juvenile hormone (e.g. Sroka
and Gilbert 1971; Pan and Wyatt 1971, 1976; Herman and Bennett 1975; Herman and
Barker 1977; Hagedorn and Kunkel 1979; Lessman et al. 1981,
Egg production), but how and when the corpora allata are activated to release
juvenile hormone are not clearly understood. There are often complex relationships
between oocyte development and several factors, including female nutrition, the
presence of a spermatophore and viable sperm, and oviposition (e.g. Benz 1969, Sasaki
and Riddiford 1984).
While there was no detectable increase in the rate of oocyte maturation for two
days after mating, three days after mating females contained more mature oocytes
than virgin females of the same age. This suggests two (non-exclusive) explanations:
1) mating provides a hormonal or mechanical stimulus to mature additional oocytes,
and 2) females utilize nutrients from spermatophores to produce more eggs. We think
that the second explanation is likely to be important in monarchs for several reasons.
Monarch spermatophores contain protein (Oberhauser 1992) which limit reproduction
in many Lepidoptera (Norris 1932, Labine 1968, Dunlap-Pianka et al. 1977).
Labeled amino acids from spermatophores are incorporated into eggs (Boggs and Gilbert
1979) and other female tissues (Wells et al. 1993) after mating, and females
that receive more spermatophore material show higher fecundity over the time that
they are degrading this male-derived material (Oberhauser 1989a, 1992). It is likely
that the effect of these nutrients could take three days, given the rate at which
oocytes are matured (Table I). There is some correspondence
between this period and the results of Boggs and Gilbert (1979), who found peak
incorporation of male-derived material into monarch eggs two to four days after
mating in one of the two matings they analyzed. Wiklund et al. (1993) also
found that peak incorporation of male-derived materials occurred three to four days
after mating in Pieris napi.
The lack of a positive effect of initial female mass on number of mature oocytes
in both treatments is consistent with previous work on monarchs (Oberhauser 1989a)
that showed no effect of female mass on lifetime fecundity. Fecundity in insects
is often correlated with mass (Suzuki 1978, Lederhouse 1981, Cushman et al.
1994 include data on Lepidoptera), but this effect should be less important with
increasing dependence on adult, rather than larval, nutrient sources for egg production
(Boggs in review). This provides further support for the importance of male-derived
nutrients to female monarchs.
Timing of mating and oocyte development
Our results support three conclusions about the relationship between the timing
of mating and oocyte maturation in monarchs. 1) Females mate for the first time
early in the process of vitellogenesis, when they have a few mature oocytes ready
to be fertilized. This suggests that females reject courting males before they have
begun to mature eggs, or that males avoid mating with these females (see
How is monarch mating different from that in other butterflies?). 2) Because
females delay mating until vitellogenesis has begun, male-derived nutrients are
likely to be more important for vitellogenesis than for other activities, such as
somatic maintenance and foraging activity. This makes sense from a resource allocation
viewpoint. Yolk is protein-rich, making male-derived nutrients likely to be important
in vitellogenesis, while sugars in the adult diet may suffice for activities such
as somatic maintenance and foraging (Boggs and Ross 1993). 3) The number of mature
oocytes at the time of a female's first mating is small relative to average daily
fecundities (Oberhauser 1989a, 1989b, and unpublished), so while the first eggs
laid after mating utilize nutrients obtained solely from larval reserves, male-derived
nutrients can supplement these reserves early in egg-laying. The high degree of
female multiple mating in monarchs (Pliske 1973, Suzuki and Zalucki 1986, Oberhauser
1989, How often do males and females mate?) results in constant
replenishment of male-derived nutrients during egg laying.
The timing of egg maturation could have important fitness implications for females.
An obvious strategy is to link egg maturation to host plant presence; it may be
costly to divert resources to egg production before oviposition is possible. In
some Lepidoptera egg maturation has been shown to be dependent on host plant access
(McNeil and Delisle 1989, McNeil 1991, Tamhankar et al. 1993). Monarchs, however,
lack a linkage of egg maturation and mating to host plant presence. Females probably
find oviposition sites easily due to the abundance of hostplants throughout monarch
summer ranges and their ability to fly long distances. Overwintering in large congregations
in sites without host plants could also uncouple mating and egg production from
hostplant presence. Females usually mate before they leave these congregations (Hill
et al. 1976, Brower 1985, Van Hook 1993, Wells et al. 1993). This allows
them to take advantage of the presence of many males; population densities of the
first butterflies to recolonize summer habitat might be low enough to make it risky
to migrate before mating.
Acknowledgements
We thank Don Alstad, Sarah Stai and Liz Goehring for help making cages, rearing,
dissecting, and observing butterflies; and Don Alstad, Carol Boggs, Dennis Frey,
Sonia Altizer, Liz Goehring, Dann Siems, Christer Wiklund and an anonymous reviewer
for comments on earlier versions of the manuscript. Research was supported by NSF
DEB-9220829 to KSO.
Return to Karen's Research Questions
References
Baker, H. G. and Baker, I. (1973). Amino acids in nectar and their evolutionary
significance. Nature 241:543-545.
Benz, G. (1969). Influence of mating, insemination, and other factors on oogenesis
and oviposition in the moth, Zeiraphaera diniana. J. Ins. Physiol.
15:55-71.
Boggs, C. L. (1981). Selection pressures affecting male nutrient investment at mating
in heliconiine butterflies. Evolution 35: 931-940.
Boggs, C.L. (1987). Ecology of nectar and pollen feeding in Lepidoptera. In Slansky,
F. Jr. and Rodriguez, J. G., (eds.) Nutritional ecology of insects, mites, and
spiders. John Wiley and sons. pp. 369-391.
Boggs, C. L. (1990). Effects of male-donated nutrients on female fitness in insects.
Amer. Nat. 136:598-617.
Boggs, C. L. and Gilbert, L. E. (1979). Male contribution to egg production in butterflies:
Evidence for transfer of nutrients at mating. Science 206: 83-84.
Boggs, C.L. and Ross, C. L. (1993). The effect of adult food limitation on life
history traits in Speyeria mormonia (Lepidoptera: Nymphalidae). Ecology
74:433-441.
Boggs, C.L., Smiley, J. T, and Gilbert, L. E. (1981). Patterns of pollen exploitation
by Heliconius butterflies. Oecologia 48:284-289.
Boggs, C. L., and Watt, W. W. (1981). Population structure of Pierid butterflies.
I. Genetic and physiological investment in offspring by male Colias. Oecologia
50:320-324.
Brower, L. P. (1985). New perspectives on the migration biology of the monarch butterfly,
Danaus plexippus L. In Rankin, M. A., (ed.), Migration: mechanisms and adaptive
significance, Austin: University of Texas, Contributions in Marine Science,
Supplement, 27, pp. 748-785.
Cushman, J. H., Boggs, C. L., Weiss, S. B., Murphy, D. D., Harvey, A. W., and Ehrlich,
P.R. (1994). Estimating female reproductive success of a threatened butterfly: Influence
of emergence time and hostplant phenology. Oecologia 99:194-200.
Drummond, B. A. lll (1984). Multiple mating and sperm competition in the Lepidoptera.
In Smith, R. L. (ed.), Sperm Competition and the Evolution of Animal Mating Systems,
Academic Press, Orlando, pp. 291-371
Dunlap-Pianka, H., Boggs, C. L., and Gilbert, L. E. (1977). Ovarian dynamics in
Heliconiine butterflies: programmed senescence versus eternal youth. Science
197:487-490.
Ehrlich, A. H., and Ehrlich, P. R. (1978). Reproductive strategies in the butterflies.
I. Mating frequency, plugging, and egg number. J. Kans. Entomol. Soc. 51:666-697.
Gilbert, L.E. (1972). Pollen feeding and reproductive biology of Heliconius
butterflies. Proc. Nat. Acad. Sci. USA 69:1403-1407.
Gilbert, L. E. (1976). Postmating female odor in Heliconius butterflies:
A male-contributed antiaphrodisiac? Science 193:419-420
Greenfield, M. D. (1982). The question of paternal investment in Lepidoptera: male-contributed
proteins in Plodia interpunctella. Int. J. Inv. Repro. 5:323-331.
Hagedorn, H.H. and Kunkel, J. G. (1979). Vitellogenesis and vitellin in insects.
Ann. Rev. Entomol. 24:475-505.
Herman, W. S. and Barker, J. F. (1977). Effect of mating on monarch butterfly oogenesis.
Experientia 33:688-689.
Herman, W. S. and Bennett, D. C. (1975). Regulation of oogenesis, female specific
protein production, and male and female reproductive gland development by juvenile
hormone in the butterfly, Nymphalis antiopa. J. Comp. Physiol. 99:331-338.
Hill, H.F., Wenner, A. M., and Wells, P. H. (1976). Reproductive behavior in an
overwintering aggregation of monarch butterflies. Amer. Midl. Natur. 95:10-19.
Labine, P. A. (1968). The population biology of the butterfly Euphydryas editha.
VIII. Oviposition and its relation to oviposition in other butterflies. Evolution
22:799-805.
Lederhouse, R. C. (1981). The effect of female mating frequency on egg fertility
in the black swallowtail, Papilio polyxenes asterius (Papilionidae). J.
Lep. Soc. 34:266-277.
Lessman, C. A, Rollins. L. and Herman, W. S. (1982). Effects of juvenile hormones
I, II, and III on reproductive tract growth in male and female monarch butterflies.
Comp. Biochem. Physiol. 71:141-144.
McNeil, J. N. and Delisle, J. (1989). Are host plants important in pheromone-mediated
mating systems of Lepidoptera? Experientia 45:236-240.
McNeil, J. N. (1991). Behavioral ecology of pheromone mediated communication in
moths and its importance in the use of pheromone traps. Ann. Rev. Entomol.
36:407-430.
Norris, M. J. (1932). Contributions towards the study of insect fertility. I. The
structure and operation of the reproductive organs of the genera Ephestia
and Plodia (Lepidoptera: Phycitidae). Proc. Zool. Soc. London. 1932:595-611.
Oberhauser, K. S. (1988). Male monarch butterfly spermatophore mass and mating strategies.
Anim. Behav. 36: 1384-1388
Oberhauser, K. S. (1989). Effects of spermatophores on male and female monarch butterfly
reproductive success. Behav. Ecol. Sociobiol. 25:237-246.
Oberhauser, K. S. (1992). Rate of ejaculate breakdown and intermating intervals
in monarch butterflies. Behav. Ecol. Sociobiol. 31:367-373.
Oberhauser, K. S. and R. Hampton. 1995. Relationship between mating and oogenesis
in monarch butterflies. J. Ins. Behav. 8:701-713.
Pan, M. L. and Wyatt, G. R. (1971). Juvenile hormone induces vitellogenin synthesis
in the monarch butterfly. Science 174:503-505.
Pan, M. L. and Wyatt, G. R. (1976). Control of vitellogenin synthesis in the monarch
butterfly by juvenile hormone. Devel. Biol. 54:127-134.
Pliske, T. E. (1973). Factors determining mating frequencies in some new world butterflies
and skippers. Ann. Ent. Soc. Amer. 68:143-151.
Rutowski, R. L., Gilchrist, G. W., and Terkanian, B. (1987). Female butterflies
mated with recently mated males show reduced reproductive output. Behav. Ecol.
Sociobiol. 20:319-322 .
Sasaki, M., and Riddiford, L. M. (1984). Regulation of reproductive behavior and
egg maturation in the tobacco hawk moth, Manduca sexta. Physiol. Entomol.
9:315-327
Sroka, P. and Gilbert, L. I. (1971). Studies on the endocrine control of postemergence
ovarian maturation in Manduca sexta. J. Ins. Physiol. 17:2409-2419.
Suzuki, Y. (1978). Adult longevity and reproductive potential of the small cabbage
white, Pieris rapae crucivora, and its role in behaviour. J. Comp. Physiol.
130:191-199.
Suzuki, Y. and Zalucki, M. P. (1986). The influence of sex ratio on female dispersal
in Danaus plexippus (L.) (Lepidoptera:Danaidae). J. Aust. Entom. Soc.
25:31-35.
Svärd, L. and Wiklund, C. (1988a). Prolonged mating in the monarch butterfly Danaus
plexippus and nightfall as a cue for sperm transfer. Oikos 52:351-354.
Svärd, L. and Wiklund, C. (1989). Size and production rate of ejaculates in relation
to monandry/polyandry in butterflies. Behav. Ecol. Sociobiol. 24:395-402.
Tamhankar, A. J., Gothi, K. K., and Rahalkar, G. W. (1993). Host-induced augmented
reproduction in spotted bollworm, Earis vittella (Fabricius) (Lepdioptera:
Noctuidae). Insect Sci. Applic. 14:371-375.
Van Hook, T. (1993). Non-random mating in monarch butterflies. In Malcolm, S. B.
and Zalucki, M. P. (eds.), Biology and Conservation of the monarch butterfly. Natural
History Museum of Los Angeles County, Los Angeles, CA, pp. 49-60.
Watanabe, M. (1988). Multiple matings increase the fecundity of the yellow swallowtail
butterfly, Papilio xuthus L., in summer generations. J. Ins. Behav.
1:17-30.
Wells, H., Wells, P. H., and Rogers, S. (1993). Is multiple mating an adaptive feature
of monarch butterfly winter aggregation? In Malcolm, S. B. and Zalucki, M. P. (eds.),
Biology and Conservation of the monarch butterfly. Natural History Museum
of Los Angeles County, Los Angeles, CA, pp. 61-68.
Wiklund, C., Kaitala, A., Lindfors, V. and Abenius, J. (1993). Polyandry and its
effect on female reproduction in the green-veined butterfly (Pieris napi
L.). Behav. Ecol. Sociobiol. 33:25-33.
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