Figure 1 Sexual dimorphism in adult males and hermaphrodites. The hypodermal cells that divide and give rise to the vulva in hermaphrodites fail to divide in the male and instead join the hypodermal syncytium. For example, the male tail is equipped with various specialized sensory and copulatory structures that enable him to locate the vulva and successfully inseminate the hermaphrodite. The hermaphrodite tail is relatively unspecialized and tapers to a thin point. All of the numerous aspects of sexual dimorphism, stemming from the earliest regulation of zygotic gene expression soon after fertilization to some of the latest events during gametogenesis in the mature adult, are controlled and coordinated by a regulatory gene hierarchy. The embryos are laid through the vulva , an opening in the ventral hypodermis that also serves as the site of entry for male-produced sperm.
This modified female is therefore able to reproduce in the absence of any other individual. The intestine is functionally specialized for yolk production only in hermaphrodites Kimble and Sharrock Dosage compensation is achieved by modulating the transcript levels of active X-linked genes, rather than by inactivating an X chromosome. For example, the male tail is equipped with various specialized sensory and copulatory structures that enable him to locate the vulva and successfully inseminate the hermaphrodite. In particular, hermaphrodites reduce transcript levels from both X chromosomes by half Meyer and Casson ; Chuang et al. However, given the opportunity, she will mate with a male, use his sperm preferentially, and produce more progeny as a consequence. Sex-specific muscles and neurons are involved in egg-laying behavior in hermaphrodites as well as mating behavior, copulation, and locomotory behavior in males. A ratio of 0. Worms with an X: Embryonic development is almost identical between the sexes. The hermaphrodites are simply self-fertile females whose only male character is the ability to make the limited number of sperm used solely for internal self-fertilization. In fact, sexual dimorphism occurs in all tissue types and arises in almost all major branches of the cell lineage Sulston and Horvitz ; Kimble and Hirsh ; Kimble and Sharrock ; Sulston et al. In each arm, the sperm are stored internally in a specialized compartment, and the oocytes are fertilized as they pass through the compartment into the uterus. The hypodermal cells that divide and give rise to the vulva in hermaphrodites fail to divide in the male and instead join the hypodermal syncytium. Extensive dimorphism also occurs in both the musculature and the nervous system. Although some adult structures such as the pharynx and the main body musculature appear to be identical between the sexes, other aspects of the anatomy and many aspects of behavior are dramatically different Fig. A ratio of 1. Diverse strategies are used to generate sexual dimorphism, including sex-specific programmed cell death, generation of sex-specific blast cells, alternative lineages adopted by a common primordium, and differential gene expression in tissues with identical cell lineages. The hermaphrodite tail is relatively unspecialized and tapers to a thin point. For example, the hermaphrodite has a two-armed gonad in which spermatogenesis sperm produced per arm occurs during the last larval stage, followed by oogenesis during adulthood. Other sexual dimorphisms arise during embryogenesis and the first three larval stages, but the differences in males and hermaphrodites become most prominent in the L4 larval stage and in the adult Sulston and Horvitz Figure 1 Sexual dimorphism in adult males and hermaphrodites. Achieving proper differentiation of the two sexes requires the coordination of a large number of developmental events that are spatially, mechanistically, and temporally distinct. This hermaphrodite-specific process is essential for the viability of XX animals Hodgkin ; Meyer and Casson ; Nusbaum and Meyer ; Plenefisch et al. The first aspect of sexual dimorphism to develop in worms is one shared by all somatic cells: A signal itself, genes that respond directly to the signal to control coordinately both sex determination and dosage compensation, and more specialized genes that regulate either sex determination or dosage compensation.
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