FACULTATIVE SEX REGULATION
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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 affect 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 and 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.
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