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FURTHER STUDIES ON THE
GENETICS OF SOLITARY AND GREGARIOUS EMERGENCE IN THE PARASITOID WASP MUSCIDIFURAX
RAPTORELLUS: PATERNAL MODIFICATION OF
LARVAL AGGRESSION. {NOTE: All Muscidifurax females regularly
superparasitize their hosts (i.e., deposit more than one egg on a host
pupa). However, in gregarious
sibling species development results in the survival of more than one
individual to the adult stage. In solitary
species only a single individual survives to the adult stage. In Holland Dr. Richard Stouthamer expanded
the study of the larvae of this study as included herein}. Summary
The
pteromalid parasitoid Muscidifurax
raptorellus Kogan & Legner
is polymorphic for its developmental behavior, fly pupae parasitized by the
solitary form always give rise to one offspring, while fly pupae parasitized
by the gregarious form give rise to more than one offspring in approximately
60% of the hosts. The study of inheritance of gregarious emergence, i.e. more
than one wasp emerges per host, in great detail. He found this trait to be polygenically controlled by 2‑19
genes. Females carrying a higher
percentage of their genome from the gregarious form have a higher percentage
of their offspring emerging gregariously from a host. More surprisingly he also found that the
father's genetic background influenced the rate at which his progeny emerged
gregariously. Males from a gregarious
line, when mated with females from a solitary line, caused the females to
start having offspring more gregariously.
The opposite effect takes place when males from the solitary line are
used. Legner's hypothesis was that
the males transferred some behavior modifying substance with their sperm to
the female, which resulted in a higher or lower level of gregarious
development. Here an alternative
hypothesis is tested in which the
level of gregarious emergence from hosts is determined by not only the number
of eggs a female oviposits in a host, but also by the level of larval
aggression. The fathers influence on
the level of gregarious development in this hypothesis takes place through
his contribution to the genetic makeup of the larvae, with a larger fraction
of their genome from the solitary developmental line showing higher levels of
larval aggression. Under these
experimental conditions the identity of the influencing the level of gregarious development by a
female, requires further examination.. ---------------------------------------------------------------------------------------------------------------------------------------- Introduction
Species
of the pteromalid genus Muscidifurax
are parasitoids of synanthropic flies.
The genus is indigenous to the Americas where it shows great diversity
(Kogan & Legner 1970). Typically
most species superparasitize with 2-3 eggs but only one wasp emerges per
host. However, in a South American
species Muscidifurax raptorellus Kogan & Legner,
two populations are known that differ in the number of wasps emerging per
housefly pupa (Legner 1987a,b). The
form originating from Chile produces offspring gregariously, i.e. from
approximately 60% of the hosts more than one wasp emerges while up to 17
wasps have been known to emerge from a single housefly pupa (Legner
1987b). However, the form originating
from Peru rarely produces more than one offspring per host. The genetics of level of gregarious
oviposition has been studied in detail by Legner (1987, 1988a,b; 1989a,b,c;
1991a,b; 1993). The level of
gregarious emergence of the offspring of a virgin female was found to be
determined by the proportion of the genome originating from the gregarious
form (Legner 1987b). The higher this
proportion the higher the level of gregariousness. This trait appears to be polygenically determined and 2‑19
genes are involved in coding for this character (Legner 1991a). More surprisingly, the level of
gregariousness in offspring emergence was not only a function of the mother’s
genetic makeup but the father's genetic background also influenced the level
of gregariousness in which his offspring emerged. Considerable amounts of data indicated that the male actually
influences the females oviposition behavior in such a way that if the male
originates from a gregarious line his mate lays her eggs more gregariously,
and female lays fewer eggs per host when she mates with males originating
from the solitary line (Legner 1987b, 1988b, 1993). High levels of variation in these experiments suggested that
other mechanisms might interplay in the paternal influencing of maternal
behavior. A number of possible
behavior modifying substances, that could be transferred from the male to the
female through the seminal fluid, have been suggested (Legner 1987b), these include
hormones and bacteria. Because males
of the solitary line reduce the gregariousness of the offspring, and males of
the gregarious line increase it one would have to assume the presence of two
different substances: one that increases the level of gregarious oviposition
and another that decreases this level. Another
hypothesis to explain level of gregarious emergence suggests that it is
determined by two factors: a) the number of eggs a female oviposits per host
and b) the level of aggressive interactions between the larvae emerging from
the eggs. The influence of the father
in this hypothesis is primarily through his genetic contribution to his
offspring (factor b), while under Legner's (1987b) original hypothesis the
number of eggs a female lays is influenced by factor a. These hypotheses were discriminated by
mating females to males either of the gregarious form or of the solitary form
and by determining in half of the
hosts the number of eggs that were oviposited and in the other half the
number of wasps that emerged. The
mothers in these tests were F1 hybrid
females between the gregarious and solitary line. These females were chosen because they are known to show both
kinds of responses; i.e. when mated to a male of the gregarious line the
females produce more offspring per host than when allowed to oviposit as
virgins (Legner 1987b), and when such hybrid females are mated to solitary
males they produce fewer offspring per host than virgin females. Materials and Methods
Culture origin: A
Peruvian/Chilean hybrid and a Chilean strain used in these experiments were
maintained on pupae of the housefly (Musca domestica
L.). The hybrid was formed by mating
large numbers of virgin Chilean
females to Peruvian males followed by porpagation of the culture for 6
generations, after which the hybrid continued to reproduce solitarily in mass
culture. Experimental
setup: All experiments were done at 25EC and 24 hrs light. Hybrid females between the solitary and
gregarious lines were secured by mating solitary females with gregarious
males. Daughters of this cross were
used in the experiment, where they were randomly assigned to one of three
treatment groups. The treatment
groups consisted of mating the females to males of either (a) gregarious or
(b) solitary strain or (c) leaving them unmated. To each group respectively 16, 13 and 21 females were
assigned. After 24hrs the males were
removed and each female was given 20 hosts daily for oviposition The hosts of each day were randomly
assigned to two groups of each 10 hosts.
The hosts of the first group were opened within 48 hrs after
parasitization had taken place to determine the number of eggs laid per host. The hosts of the second group were
individually placed in a gelatin capsule (size 000) for wasp emergence. In total the parasitization of these wasps
was followed for 5 days, thus per mother 50 hosts were opened to determine
the number of eggs laid per host and 50 hosts were kept for emergence. The
experiment was done in such a way that the mother's group identity, i.e.
whether she was mated to a solitary male or a gregarious male, was not known
to the person who did the egg and wasp counts. This avoided prior knowledge from influencing the results. Analysis
of variance was performed on the data derived from the two samples of fifty
hosts that were either opened for egg counts or were left for wasp emergence
for each mother: the percentage of the hosts that had been opened containing
more than one egg, the percentage of hosts from which more than one wasp
emerged, the difference between these two percentages, the total number of
eggs in the egg sample, the total number of wasps in the wasp sample and the
difference between the total number of eggs and the total number of wasps,
the total number of parasitized hosts, i.e. those containing one or more
eggs, the total number of parasitized hosts resulting in adult offspring and
the difference between these two measurements per female. No transformation was done for the statistical
analysis of the data. To determine
the significance of the differences between means a Duncan's Multiple range
test was used with alpha =0.05. Results
The
results shown in table 1 indicate that the percentage of hosts in which more than one egg
was found did not differ significantly between females mated with a male from
the gregarious line or the solitary line.
There was a significant difference however between females mated to solitary
males versus virgin females in the proportion of the eggs that were placed
gregariously on a host. The
percentage of wasps that emerged gregariously did however differ
significantly between all groups with that percentage being the highest for
the offspring of the gregarious male, intermediate for the offspring of the
virgin females and the lowest for the offspring of the solitary males. The
difference between the percentages of hosts with gregarious eggs vs
gregarious wasps also differed between the three groups. The group mated to the gregarious males
showed the smallest difference, the virgin females were intermediate while
the offspring of the solitary males showed the largest reduction in the
percentage gregariousness. There
were no significant differences between the groups in the total number of
eggs per 5 day sample (table 2), however the number of wasps that emerged from the five day
sample did differ significantly: the gregarious group had a significantly
higher number of adult progeny than the solitary or virgin group. These two
groups however did not differ from each other in this characteristic. The difference between the number of eggs
per five day sample and the number of wasps per sample differed among all
groups with the gregarious group showing the smallest reduction in numbers
and the solitary the highest. Finally
there appeared to be no difference in the number of hosts that contained eggs
between the groups nor in the number of hosts giving rise to wasps nor in the
difference between the number of hosts containing eggs versus those giving
rise to wasps (table 3). Discussion
In the
current experiments the number of eggs laid per host by a hybrid female was
not appreciably influenced by the type of male to which she was mated. However, the genetic background of the
male did influence the percentage of hosts from which more than one wasp
emerged. Consequently, the hypothesis
that the male influences the female’s egg laying behavior has to be rather in
favor of the hypothesis that larvae with a large proportion of their genome
stemming from the solitary line somehow reduce the number that will
eventually emerge from a host. The
number of eggs laid per mother did not differ between the groups, and neither
did the number of hosts that has been parasitized. Therefore, the reduction in the number of hosts from which
wasps emerge gregariously was caused by a reduction during the larval stage. Clearly, the offspring of the solitary
males had the highest mortally, followed by that of the virgin females and
the least mortality took place in the offspring of the gregarious males. Several authors have reported that
aggressive interactions take place between the larvae of the species M. raptor Girault & Sanders. Wylie (1971) found that larvae of M. raptor
will spend approximately the first ten hours after hatching moving about on
the host while occasionally feeding on the pupa. If a host has several eggs on it, the larva that hatches first
attacks and kills most or all of the eggs.
If two larvae happen to survive the first instar, combat will take
place in the second instar. Podoler
and Mendel (1977) report similar observations for M. raptor. Legner (1987b) also reports that <2% of
the hosts parasitized by the solitary form of M. raptorellus
receive more than one egg, but rarely more than one wasp emerges. This could also indicate that larval
aggression occurs in this species and indeed aggression was observed between the
larvae of the solitary form of this species.
Apparently the larval combat generally leads to only one wasp emerging
per host, (table 3). The level
of aggressiveness between the larvae can be influenced to a maximal extent
when males of the different lines are crossed with hybrid females between
these lines. The fertilized eggs
result in female larvae that have on average either 75% of their genome from
the solitary line ((S & X G %)& X S %)& or 25% of the solitary
line ((S& X G%)& X G%)&.
In the first case the larvae apparently engage in combat which results
in a reduction in the percentage of hosts that give rise to more than one
offspring (fig. 1c). While in the
second case the larvae show less aggressive interactions, and per host, more
of the eggs survive to give rise to adult wasps (fig. 1a). The offspring of the virgin females is
intermediate because all of their offspring will have on average 50% of their
genome stemming from the solitary line (fig 1c.). There could also be an influence of the father on the survival
of male and female larvae in these crosses.
In the case of solitary fathers the fertilized eggs, i.e. daughters
have on average 75% of the genome of the solitary line and can be expected to
fight more than their male siblings, which have 50% of their genome from the
solitary line. In the case of
offspring of the gregarious males, their daughters have only 25% of their
genome coming from the solitary line while the males have 50% of their genome
coming from the solitary line.
Consequently, in those cases where one male and one female are placed
in a host in case of the "gregarious" group it would be expected
that the one male one female combination would be more prevalent than in case
of the "solitary" group, while at the same time the one male group
should be larger in the gregarious line than in the solitary line. Although
indeed these trends are visible in the data (table 4) they are rather subtle,
probably mainly because the fraction of the hosts receiving one male and
female egg is limited in these crosses. The
gregarious emergence is the result of two, possibly linked, effects: the
number of eggs a female oviposits per host, this effect is determined purely
by the female’s genome and secondly the level of aggressive interactions
between the larvae, this level is influenced by the genetic background of
both parents. Females of the solitary
line lay fewer eggs per host than the females of the gregarious line. It is suspected that larvae of the
gregarious line show little aggressive behavior towards each other, while the
larvae of the solitary line are aggressive. The gregarious strain also
superasitizes: that is, more eggs are
laid in a hsot than will survive to adulthood. The cause of the mortality in the ones that do not survive
could be due to genetic inferiority or to some degree of aggressiveness.
These two groups of genes could interact in such a way that they would
enhance each other. The higher the
percentage of the genome stemming from the gregarious line the higher the
number of hosts with more than one egg and the lower the level of larval
aggression, in contrast the lower the percentage of the gregarious genome the
lower the level of gregarious oviposition and the higher the level of larval
aggression. Therefore, one would
expect a slight s shaped curve to describe the relationship between
the percentage of the genome from the gregarious line in a virgin mother and
the percentage of the hosts giving rise to more than one offspring. When this relationship (Legner 1987b,
1991a) is inspected indeed such a deviation, albeit slight, is found. The findings of Legner (1987b, 1991a) on the
number of genes influencing the gregarious and solitary oviposition should be
reexamined in the light of these two groups genes being involved. The finding that within one species both
solitary, fighting forms and gregarious, non‑fighting forms appears to
be unique to M. raptorellus. This will allow the testing of some of the
theory developed by Godfray (1987) on the evolution of solitary and
gregarious oviposition. There is
also a heterosis in the F-1 hybrids that is not explained in this
discussion. The heterosis is
expressed in several ways: higher
numbers of offspring and hosts killed, etc.
Also, backcrossing data suggest that at least eight loci were actively
segregating for gregariousnes behavior. References
Godfray, H. C. J. 1987. The evolution of
clutch size in parasitic wasps. Am. Nat. 129: 221‑233 Legner, E. F. 1987a.
Further insights into extranuclear influences on behavior elicited by
males in the genus Muscidifurax. Proc. 52 annual conf. Cal. Mosquito and Vector Control
Association: 127‑130. Legner, E.
F. 1987b.
Inheritance of gregarious and solitary development in Muscidifurax raptorellus. Can. Ent. 119: 791‑808. Legner, E. F. 1988a.
Hybridization in principal parasitoids of synanthropic diptera: The
genus Muscidifurax. Hilgardia 56(4): 36pp. Legner, E.
F. 1988b. Muscidifurax raptorellus females exhibit
postmating oviposition behavior typical of the male genome. Ann. Entomol.
Soc. Am. 81: 522‑527
Legner, E.
F. 1989a.
Wary genes and accretive inheritance in Hymenoptera. Ann. Entomol. Soc. Amer. 82: 245‑249.
Legner, E. F.
1989b. Paternal influences in
males of Muscidifurax raptorellus. Entomophaga
34: 307‑320 Legner, E.
F. 1989c.
Phenotypic expression of polygenes in Muscidifurax raptorellus,
a synanthropic fly parasitoid. Entomophaga
34: 523‑530. Legner. E. F.
1991a. Estimations of
number of active loci, dominance and heritability in polygenic inheritance of
gregarious behavior in Muscidifurax
raptorellus. Entomophaga
36: 1‑18 Legner, E.
F. 1991b.
Recombinant males in the parasitic wasp Muscidifurax raptorellus.
Entomophaga 36: 173‑181 Legner, E.
F. 1993.
Theory for quantitative inheritance of behavior in a protelean
parasitoid, Muscidifurax raptorellus (Hymenoptera: Pteromalidae). Eur. J. Entomol. 90: 11-21. Podoler, H.
& Z. Mendel. 1977. Analysis of solitariness in a
parasite-host system (Muscidifurax raptor – Ceratitis capitata). Ecol. Entomol
2: 153-160 Wylie, H.
G. 1971.
Observations on intraspecific larval competition in three
hymenopterous parasites of fly puparia. Can. Ent. 103: 137‑142 -------------------------------------------------------------------------------------------------------------------------------------- Table 1. Mean and standard error of the percentage of hosts that
contained more than one egg (egg/hst), from which more than one wasp emerged
(wasp/hst), and the difference between these values for hybrid females that
had mated either with a gregarious male, had remained virgin or had mated
with a solitary male. n >egg/hst >wasps/hst difference gregarious male 16 38.4
(2.39) 28.7
(2.13)
9.8 (2.93) virgin 21 33.8 (2.08) 12.3 (1.86) 21.6 (2.56) solitary male 13 43.1 (2.65) 5.3 (2.36) 37.8
(3.25) Table 2. Mean
and standard error of the total number of offspring (eggs or wasps) and the
difference between these numbers, for hybrid females that had mated either
with a gregarious male, had remained virgin or had mated with a solitary
male. n >egg/hst >wasps/hst difference gregarious male 16 60.6
(2.25) 54.8 (1.97) 5.8
(1.95) virgin 22 57.0 (1.97) 43.2
(1.72) 13.8
(1.71) solitary male 13 58.8 (2.51) 38.7
(2.19) 20.1
(2.17) Table 3. Mean and standard error of the number of parasitized hosts
indicated by the presence of
one or more eggs, by the emergence of one or more wasps and the difference
between these numbers, for hybrid females that had mated either with a
gregarious male, had remained virgin or had mated with a solitary male. n hosts with egg host
with wasps difference gregarious male 16 41.6 (1.24) 40.3
(1.40)
1.3 (0.93) virgin 21 40.9 (1.08) 38.9 (1.22) 2.1 (0.81) solitary male 13 39.2 (1.37) 37.3 (1.55) 1.9 (1.03) Table 4. Total egg and wasps
distribution per host for females that had either mated with males from a
gregarious line, had remained virgin or had mated with males from a solitary
line. Egg allocation per host,
represented as the percentage of all hosts in egg sample containing either 1,
2, 3 or 4 eggs. Wasp distribution per
hosts represented as the percentage of hosts containing a male (M) or a
female (F) or a combination of the two. No. Sample dissected for egg counts of mothers --------------------------------------------------------- % of hosts with 1-4 eggs
No. 1 2 3 4 hosts gregarious male 16 677 62.5 32.2 5.0 0.3
virgin 22 855 65.1 30.4
3.7 0.7 solitary male 13 507 56.8 36.9 5.3 1.0 No. Sample left for wasp counts of ----------------------------------------------------------------- mothers % of
hosts from which males and/or females emerged No. 1M 1F 2M 2F 1M1F 1M2F 3M 3F hosts gregarious male 16 661 10.8 59.6
0 23.1 3.5
0.3 0 2.6
virgin 22 803
87.5 0 12.1 0 0 0
0.4 0 solitary male 13 483 9.3 86.1 0 3.5 1.0 0 0 0 |