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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. Summary
The pteromalid parasitoid Muscidifurax raptorellus
Kogan & Legner is polymorphic for its oviposition 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. Legner has studied the 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 oviposition. 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 emergence in this hypothesis only takes place through his
contribution to the genetic makeup of the larvae, with the larvae with a larger
fraction of their genome from the solitary line showing higher levels of
larval aggression. Under our
experimental conditions the identity of the male did not influence the level
of gregarious oviposition by a female, but it did have a significant influence
on the number of larvae that survived.
The maternal behavior modification hypothesis did not apply
significantly in this case. ---------------------------------------------------------------------------------------------------------------------------------------- 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 lays 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
oviposition 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 |