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FACULTATIVE SEX REGULATION
In Arthropods
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Spermatheca
and Spermathecal Gland
Types
of Spermathecae in Hymenoptera
References [ Please refer also to Selected Reviews & Detailed
Research ]
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History Biparental Hymenoptera are well known
for their ability to regulate the sex ratio of their offspring, which is
dependent on their haplo-diploid kind of reproduction. In the process, the
ovipositing female responds by instinct to environmental stimuli. Stimuli
such as host density, host size, temperature, humidity, parasitoid density,
etc., interact on the instincts of the female to cause her to lay fertilized
or unfertilized eggs. However, the female's physiological condition can
effect the way she responds to various stimuli. When such factors as lack of
mating, female age and nutrition, infection with certain microorganisms,
genetic phenomena, etc. are involved, sex regulation is not possible. Spermatheca and
Spermathecal Gland The spermatheca and spermathecal
gland are organs that enable sex regulation to take place. Sperm stored in
the hymenopteran spermatheca are quiescent except when the females are in
contact with oviposition sites which stimulates the spermathecal gland
(Flanders 1939). Flanders demonstrated sperm quiescence in the wasp Tiphia,
which had been suspected by Lillie (1919, p. 132). To account for this
quiescence, Flanders accepted as valid the conclusions of Lillie (1919) and
other contemporaries that sperm when in concentrated suspensions retain their
vitality longer than in more dilute suspensions because they rapidly produce
a paralyzing concentration of CO2. Lillie (1919) noted that insect
sperm when stored in the spermatheca retain their vitality for many years. The few sperms at the opening of
the spermatheca must be subjected to some activating agent before the egg can
be fertilized. The source of such an agent appears to be the spermathecal
gland, which presumably secretes a fluid that is slightly alkaline (Flanders
1946). In the honeybee, Lensky & Schindler (1967) found that the
spermathecal gland fluid and contents of the spermatheca ranged from neutral
to pH 9; and the activation of sperm was accomplished by dilution in pH 4.5
to 9.0. This refutes Lillie's hypothesis of CO2 anesthetization. The capacity of the spermathecal gland to keep pace with
the rate of egg deposition is probably an important factor in determining the
sex ratio (Flanders 1947). Types of
Spermathecae in Hymenoptera.--Many hypotheses antedated Flanders (1939) to explain
the apparent ability of the mated arrhenotokous female to control the action
of the spermatheca. Phillips (1903) made some remarks about facultative
fertilization, and Schrader (1920) stated that the female fertilizes its eggs
in response to a stimulus, while studying the white fly Trialeurodes vaporariorum
(Westwood). Flanders believed that the spermathecal gland may be stimulated
by contact of the antennae, legs, or ovipositor with the medium on which the
eggs are to be deposited, with variations in the condition of this medium
causing variable amounts of stimulation. It is probable that the spermatheca
responds only to the stimuli of certain intensities, reacting to its fullest
extent or not at all. In some biparental species the spermatheca, if
functional, may be stimulated only by the passage of an egg. In such cases
every deposited egg would be fertilized as long as the spermatheca contained
viable sperm. It is interesting that in some females that reproduce by
thelytoky, the spermatheca appears capable of functioning. Sometimes mating
may even occur in such species [e.g., Encyrtus spp., Muscidifurax
uniraptor Kogan & Legner, Aphytis mytilaspidis
(LeBaron) (Legner 1988a, Rossler & DeBach 1972a)]. Significant behavioral and
sex ratio changes accompany mating of thelytokous females, however (Legner 1988b). A peculiar
situation is known to exist with some thelytokous Coleoptera [e.g., the
white-fringed beetle, Graphognathus leucoloma (Boheman)] where
copulation with a male of a different species is required before any of the
thelytokous eggs can be viable. Three types of spermathecae have
been described in Hymenoptera (Flanders 1939, 1956). Type I
is found in the honeybee, and Tiphia. There is a wide sperm duct
which can be bent into a valve at its juncture with the capsule. A number of
sperm can be discharged simultaneously, and glandular fluids activate and
transport sperm, which serves to regulate the number of sperm released on
each egg as it passes along the oviduct. The spermathecal gland empties into
the sperm capsule instead of the sperm duct as in other species. Type II spermatheca is found in ichneumonids and braconids. The
spermathecal gland empties into the lumen of the sperm duct. The short exit
passage from the capsule to the sperm duct is so narrow that only a single
sperm can move in or out at one time. The gland is very voluminous in
ichneumonids, while braconids have a smaller gland but it is accompanied by a
contractile reservoir. Type III spermatheca occurs in chalcidoids. The gland is very
small, and glandular secretions serve only for sperm activation. The sperm in
these species are extraordinarily long. Considerations in
Sex Regulation In females with activated sperm,
the sequence of fertilized and unfertilized eggs may be determined in part by
the rate of oviposition. The number of sperms present in the spermatheca can also
influence sex ratio as well as previous oviposition experience. Marchal
(1898) suggested that the power of discrimination on the part of the female
is effected through the differential stimulation of the spermatheca, the
latter being activated only when the female honeybee oviposits in large
cells. Marchal also was the first to suggest that a spermatheca could become
fatigued. Coccophagus ochraceus Howard parasitizing Saissetia
spp. stands on top of the host when her spermathecal gland is turgid and injects
a fertilized egg internally. When the gland is depleted she stands at the
side of the host and deposits a male egg underneath the host in a dry
environment. In endoparasitic braconids of the
genera Apanteles and Opius, the females are characterized by
small uterine-stored eggs that can be deposited very rapidly. Since these
females possess Bracon-type spermathecae, the spermathecal gland of
gregarious species cannot keep pace with the egg deposition. The proportion
of eggs that thus escape fertilization is so great in Apanteles and Opius
(Biosteres) that males usually predominate even at low densities under
field conditions. Spalangia species deposit more fertilized eggs when
oviposition occurs at high than at low host densities (Legner 1967a, 1967b), a trend that was also observed in Goniozus legneri
Gordh (Legner & Warkentin 1988 ). However, the rate of oviposition here is not rapid in a
way that some eggs might escape fertilization. External stimuli (namely
variable host densities) are thought to influence the spermathecal gland
directly. This response of fertilizing more eggs at high host densities was
hypothesized by Flanders (1939) using evidence from field data, where sex
ratios tend to favor females at increasing host field densities. Exercise 19.1--How do sperm stored in the spermatheca become activated? Exercise 19.2--Describe the various types of spermathecae in
Hymenoptera. Exercise 19.3--How does rate of oviposition influence the sex ratio? Exercise 19.4--How does host density influence the sex ratio? Exercise 19.5--Explain how the progeny increase in response to host
density with parasitoids of synanthropic flies, affects the functional and
numerical responses in their populations. Exercise 19.6--Using examples of Spalangia cameroni, S.
endius, Muscidifurax and Nasonia, discuss all the subtle
responses of these species to fluctuating host densities. Explain how any or
all of such responses might be measured in nature. REFERENCES: [Additional references may be found at MELVYL Library ] Bellows,
T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook of Biological Control:
Principles and Applications. Academic Press, San Diego, CA. 1046 p. Flanders,
S. E. 1939. Environmental control of sex in hymenopterous insects. Ann. Ent.
Soc. Amer. 32: 11-26. Flanders,
S. E. 1943. The role of mating in the reproduction of parasitic Hymenoptera.
J. Econ. Ent. 36: 802-03. Flanders,
S. E. 1946a. The mechanism of sex control of the honey bee. J. Econ. Ent. 39:
379. Flanders,
S. E. 1946b. Control of sex and sex-limited polymorphism in the Hymenoptera.
Quart. Rev. Biol. 21: 135-43. Flanders,
S. E. 1947. Elements of host discovery exemplified by parasitic Hymenoptera.
Ecology 28: 299-309. Flanders,
S. E. 1956. The mechanisms of sex ratio regulation in the parasitic
Hymenoptera. Insectes Sociaux 3: 325-34. Flanders,
S. E. 1969. Herbert D. Smith's observation on citrus blackfly parasites in
India and Mexico and the correlated circumstances. Canad. Ent. 101: 467-80. 45. Legner, E. F. 1967a.
Two exotic strains of Spalangia
drosophilae merit consideration in
biological control of Hippelates collusor (Diptera:
Chloropidae). Ann. Entomol. Soc.
Amer. 60(2): 458-462. 46.
Legner, E. F. 1967b.
Behavior changes the reproduction of Spalangia cameroni, S. endius,
Muscidifurax raptor, and Nasonia vitripennis (Hymenoptera: Pteromalidae) at increasing fly host
densities. Ann. Entomol. Soc. Amer.
60(4): 819-826. 237.
Legner, E. F. 1988a.
Muscidifurax raptorellus (Hymenoptera:
Pteromalidae) females exhibit postmating oviposition behavior typical of the male
genome. Ann. Entomol. Soc. Amer.
81(3): 522-527. 241.
Legner, E. F. 1988.
Studies of four thelytokous Puerto Rican isolates of Muscidifurax uniraptor [Hymenoptera: Pteromalidae]. Entomophaga 33(3); 269-280. 240.
Legner, E. F.
& R. W. Warkentin. 1988. Parasitization of Goniozus legneri
(Hymenoptera: Bethylidae) at increasing parasite and host Amyelois transitella (Lepidoptera: Phycitidae) densities. Ann. Entomol. Soc. Amer. 81(5): 774-776. Lensky,
Y. & H. Schindler. 1967. Motility and reversible inactivation of honeybee
spermatozoa in vivo and in vitro. Ann. del Abeille 10(1): 5-16. Lillie,
F. R. 19l9. Problems of fertilization. Univ. of Chicago Sci. Series. 278 p. Marchal,
P. 1898. Le cycle evolutif de l' Encyrtus fusicollis. Bull.
Soc. Ent. de France: 109-11. Marchal,
P. 1904. Recherches sur la biologie et le developpement des hymenopteres
parasites. I. La Polyembryonie specifique ou germinogonie. Arch. de Zool.
Exp. et Gen. 2: 257-335. Marchal,
P. 1936. Recherches sur la biologie et le developpement des Hymenopteres
parasites: Les Trichogrammes. Ann. Epiphyties, Paris 2: 447-550. Phillips,
E. F. 1903. A review of parthenogenesis. Proc. Amer. Philos. Soc. 42:
275-345. Rössler,
Y. & P. DeBach. 1972a. The biosystematic relations between a thelytokous
and an arrhenotokous form of Aphytis mytilaspidis (LeBaron)
[Hymenoptera: Aphelinidae]. I. The reproductive relations. Entomophaga 17:
391-423. Rössler,
Y. & P. DeBach. 1972b. The biosystematic relations between a thelytokous
and an arrhenotokous form of Aphytis mytilaspidis (LeBaron)
[Hymenoptera: Aphelinidae]. 2. Comparative biological and morphological
studies. Entomophaga 17: 425-35. Rössler,
Y. & P. DeBach. 1973. Genetic variability in a thelytokous form of Aphytis
mytilaspidis (LeBaron) [Hymenoptera: Aphelinidae]. Hilgardia 42(5):
149-76. Schrader, F. 1920. Sex determination in the white fly (Trialeurodes
vaporariorum). J. Morph. 34: 267-98. |