Biomedical subjects
H F Nijhout
Publications and source records attributed to H F Nijhout.
Metaphors and the role of genes in development.
In describing the flawless regularity of developmental processes and the correlation between changes at certain genetic loci and changes in morphology, biologists frequently employ two metaphors: that genes 'control' development, and that genomes embody 'programs' for development. Although these metaphors have an admirable sharpness and punch, they lead, when taken literally, to highly distorted pictures of developmental processes. A more balanced, and useful, view of the role of genes in development is that they act as suppliers of the material needs of development and, in some instances, as context-dependent catalysts of cellular changes, rather than as 'controllers' of developmental progress and direction. The consequences of adopting this alternative view of development are discussed.
Independent development of homologous pattern elements in the wing patterns of butterflies.
Rank correlation analyses demonstrate that the degree of color pattern development in each wing cell of Cercyonis pegala (Satyridae) and Smyrna blomfildia (Nymphalidae) is either weakly or not at all correlated with that in other wing cells. There is much greater individual variability in pattern development in different wing cells than there is in homologous wing cells on opposite wings. This finding indicates that differences in pattern development in adjacent wing cells are not due to developmental noise, but are programmed, and that pattern development in each wing cell is in large measure independent of that in other wing cells.
Cautery-induced colour patterns in Precis coenia (Lepidoptera: Nymphalidae).
Cautery of the dorsal hindwing in the butterfly, Precis coenia, induces the formation of a concentric colour pattern around the site of injury. The induced pattern is identical in pigmentation to the eyespots that normally develop on this wing surface. This response to cautery also occurs, though much less dramatically, on the ventral forewing. In addition to the peculiar response to cautery, the dorsal hindwing of Precis also develops a series of unique pattern aberrations in response to coldshock. These consist of irregular elongation of the anterior eyespot along the proximodistal axis of the wing. In the most dramatic aberrations the eyespot field covers the entire anterior half of the wing surface. An analysis is presented that attempts to reconcile the effects of cautery on the Precis hindwing with the very different morphological effects of cautery on the colour pattern of Ephestia kühniella, described by Kühn & Von Engelhardt. Computer simulations reveal that the finding presented in this paper, as well as the classical work on Ephestia, can both be explained by assuming that the site of cautery becomes a sink for one of the morphogens involved in colour pattern determination. The experimental findings furthermore indicate that minor perturbations of the wing epidermis can evoke the physiological conditions that attend normal eyespot determination. It is shown that this interpretation also helps to explain the unusual pattern modifications following coldshock.
Colour pattern modification by coldshock in Lepidoptera.
When young pupae of Vanessa cardui, V. virginiensis and Precis coenia (Lepidoptera: Nymphalidae) are exposed to severe coldshock many develop aberrant adult wing patterns. For each species, a synchronous cohort of experimental animals always develops a broad range of aberrant pattern morphologies but these can always be arranged in a single unbranched morphological series. When such phenotypic series are compared, between species and between wing surfaces within a species (each wing surface usually bears a different colour pattern), many parallel modifications and trends become evident. These parallelisms reveal certain homologies of pattern elements and suggest that a common physiology underlies the development of a considerable diversity of normal and aberrant colour patterns. The case is made that the phenotypic series produced may represent either a series of sequential stages in colour pattern determination or, a series of quantitatively different 'interpretations' of an established gradient system. Colour pattern morphoclines reveal which patterns are developmentally 'adjacent' to one another and may therefore prove useful in elucidating the evolution of patterns.
Ultrastructural changes accompanying secretion and cell death in the molting glands of an insect (Oncopeltus).
During the fifth (last) larval instar of Oncopeltus fasciatus, morphological changes in the molting glands associated with ecdysone secretion include a increase in cytoplasmic volume relative to that of the nucleus, increased amounts of rough endoplasmic reticulum and mitochondria, and the formation of deep infoldings of the plasma membrane. On the sixth day of the fifth instar large electron-lucent areas become apparent beneath the basement membrane; however, the glands remain intact until the seventh (last) day of the instar when a dramatic fragmentation of the cytoplasm, and condensation and fragmentation of the nucleus are observed. It is likely that such changes occur rapidly, just prior to the time of ecdysis to an adult. Cell death in the molting glands of Oncopeltus is markedly different from that described for the molting glands of other insect species in that autophagic vacuoles are not observed prior to a complete loss of cellular integrity.
Pattern formation on lepidopteran wings: determination of an eyespot.
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Ontogeny of the color pattern on the wings of Precis coenia (Lepidoptera: Nymphalidae).
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Antennal hair erection in male mosquitoes: a new mechanical effector in insects.
Male Anopheles mosquitoes erect their antennal hairs prior to mating. The erectile mechanism resides in a unique annulus at the base of each hair whorl. It appears that the insect regulates the degree of hydration of this annulus. When the annulus is made to swell the attached hairs are pushed to their erect position.
Permanent loss of male fecundity following sperm depletion in Aedes aegypti (L.).
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Excretory role of the midgut in larvae of the tobacco hornworm, Manduca sexta (L.).
Caterpillars of Manduca sexta use two distinct transport mechanisms for the excretion of dyes. One pump (Type A) has a high affinity for acid (anionic) dyes and occurs in the midgut and medial Malpighian tubules. Acid dyes accumulate rapidly in the lumen of the midgut while the Malpighian tubules appear to play only a minor role in the excretion of these dyes. The other pump (Type B) excretes basic (cationic) dyes and is located primarily in the proximal Malpighian tubules. Evidence is presented that hippuric acid competes with acid dyes for excretion by both midgut and Malpighian tubules. After the final-instar larva purges its gut the ability of the midgut and Malpighian tubules to excrete dyes gradually decreases. Sixty hours after the purge only the Malpighian tubules retain some dye excreting activity.
Dynamics of juvenile hormone action in larvae of the tobacco hornworm, Manduca sexta (L.).
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A threshold size for metamorphosis in the tobacco hornworm, Manduca sexta (L.).
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Control of moulting and metamorphosis in the tobacco hornworm, Manduca sexta (L.): growth of the last-instar larva and the decision to pupate.
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Control of moulting and metamorphosis in the tobacco hornworm, Manduca sexta (L.): cessation of juvenile hormone secretion as a trigger for pupation.
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