1. Insect Physiology Developmental Systems Department of Entomology National Chung Hsing University

2. CONTENTS Preembryonic developmentPreembryonic development 胚胎前發育 Preembryonic development Insect eggs –Egg membranes 卵膜 Egg membranesEgg membranes –Pattern formation within the oocyte 卵決定作用 Pattern formation within the oocytePattern formation within the oocyte Embryonic development –Blastoderm formation 囊胚形成 Blastoderm formationBlastoderm formation –Formation of the germ band 胚帶形成 Formation of the germ bandFormation of the germ band –Blastokinesis and dorsal closure 轉胚動 Blastokinesis and dorsal closureBlastokinesis and dorsal closure –Formation of the gut 腸化作用 Formation of the gutFormation of the gut –Formation of the nervous system 神經系統形成 Formation of the nervous systemFormation of the nervous system –Formation of internal organs 內部構造形成 Formation of internal organsFormation of internal organs –Formation of the reproductive system 生殖系統形成 Formation of the reproductive systemFormation of the reproductive system –Endocrinology of embryonic development 內分泌學 Endocrinology of embryonic developmentEndocrinology of embryonic development

3. Preembryonic Development All multicellular organisms begin their lives as single-celled zygotes that divide to form groups of cells that show progressive morphological changes. Two main processes occur in the embryonic development – –Determination 決定作用 : the commitment process that establishes the later differentiated state – –Differentiation 分化作用 : the development of morphological differences and the generation of cellular diversity

4. Insect Egg Structure

5. Egg Membranes Chorion 卵殼 : synthesized within the ovariole by the follicular epithelium – –Vitelline envelope 卵黃膜 : an inner noncellular membrane with a thickness of about 0.3  m – –Wax layer 腊層 : 5 nm to 2  m; to provide protection against desiccation – –Chorionic layers: Inner chorionic layer Endochorion: inner and outer endochorion Exochorion

6. Cross-Section of an Drosophila Egg

7. Specialized Structures of Insect Eggs Micropyle 受精孔 : an opening of chorion that allows a single sperm to enter. (some with many micorpyles) Respiratory appendages: structures that serve as a plastron to extract oxygen from water. Operculum 卵蓋 : a cap that is surrounded by hatching regions and opens to allow the larva to exit.

8. Cross-Section of an Insect Egg

9. Respiratory Horns of Drosophila Egg

10. Pattern Formation within the Oocyte There are distinct regions with an oocyte – –Periplasm: cytoplasm immediately beneath the plasma membrane – –Spatial gradient of protein that result from the transcription of specific genes to create a pattern that provides the developing embryo with positional information within the egg. Morphogens

11. Polarity Gradients of Morphogens Polarity gradients of these materials are established early during oogenesis, while the oocyte resides in the female even before fertilization and oviposition occur (next slide)next slide – –Maternal effect genes: establish an anterior-posterior polarity in the egg, such as bicoid, nanos, hunchback, and caudal

12. Cross-Section of an Insect Egg

13. Fig. The mechanism of Gurken signaling to establish anterior/posterior and dorsal/ventral commitment of follicle cells.

14. Fig. The gradients of bicoid, nanos, hunchback, and caudal mRNA that establish position in the Drosophila oocyte.

15. Insect Egg Prior to Fertilization The oocyte, which has been arrested in metaphase of the first meiotic division, continues to mature after oviposition occurs. Shortly after the sperm penetrates the egg and oviposition take place, the oocyte completes meiosis.

16. Insect Egg Prior to Fertilization Syngamy: the union of sperm pronucleus and oocyte pronucleus; occurs at the interior of the egg. Parthenogenesis: reproduction without sperm involvement; a haploid polar nucleus combines with the haploid oocyte nucleus to restore the diploid.

17. Egg Cleavage Holoblastic cleavage: animals that have relatively little yolk are able to undergo complete cleavage. (e.g. mammals) Meroblastic cleavage: the relatively large amount of yolk prevents the first cleavage divisions from cutting through the entire egg, and their cleavage is more superficial cleavage. (e.g. insects)

18. Blastoderm Formation Energid: each nucleus surrounded by a small island of cytoplasm during the meroblastic cleavage of most insects After a series of mitoses, the energids migrate to the egg periphery and continue to divide there. Syncytial blastoderm: lacks any membranes, with all the cleavage nuclei contained within the common cytoplasm. Pole cells: some of the energids migrate to the posterior of the egg; give rise to the germ cells. Vitellophages: some energids remain in the yolk; involved in the digestion of yolk and the formation of the midgut epithelium.

19. Fig. Migration of energids to the periplasm of an insect oocyte, forming the syncytial blastoderm.

20. Fig. Formation of the cellular blastoderm from the syncytial blastoderm.

21. Formation of the Germ Band Embryonic primordium: the thickened portion of the blastoderm; develop into the embryo Extraembryonic ectoderm: the other cells of the blastoderm beside the primordium Germ line: the embryonic primordium increases in length; represents the ventral region of the future body

22. Fig. Development of the embryonic permordium and extraembryonic ectoderm from the blastodermal cells.

23. Gastrulation Gastrulation: the germ band develop to become a double-layer embryo The cells of the germ band migrate inward into yolk to form a multicellular layer Gastral groove: the longitudinal furrow that is formed by the elongation and migration upward of the cells along the ventral line midline of the germ band Mesoderm: the inner layer of cells of the gastal groove. The germ band constitutes the trunk of the developing insect

24. Fig. Formation of mesoderm from the migration of ectodermal cells.

25. Origin of the Mesoderm from Cells Expressing Twist

26. Segmentation Segmentation begins as the maternal effect genes activate or repress the gap genes to establish regions of segment identity. The concentrations of Bicoid, Hunchback, and Caudal proteins determine the transcription patterns of the gap genes. The produces of the gap genes regulate the expression of the pair-rule genes. The produces of the pair-rule genes further divide the broad gap gene regions into individual parasegments. (next slide)next slide

27. Segmentation The parasegments will ultimately produce the anterior compartment of one segment and the posterior compartment of the next. The segment polarity genes assure that certain repeated structures appear in each segment, establishing the cell fates within each of the parasegments. Then, subsequent interactions of the gap and pair-rule genes regulate the homeotic genes that establish the identity of each segment and its characteristic structure. (next slide)next slide

28. Blastokinesis and Dorsal Closure Amniotic folds: the folds which are formed while the germ band elongates and widens, carrying the margin of the extraembryonic ectoderm with it. – –Amnion: the inner walls of the amniotic folds. – –Amniotic cavity: the amnion enclosure – –Serosa: the ectoderm that is detached from the germ band Blastokinesis: the movements of the germ band within the yolk; reverse the relative positions of the yolk and the embryo – –Dorsal closure: after blastokinesis and growth of the embryonic ectoderm over the dorsal portion and end up with to enclose the yolk within the embryo.

29. Fig. The formation of the amnion and the invagination of the germ band to free it from the serosa.

30. Fig. The formation of neuroblasts from ectodermal tissue and the proliferation of mesoderm.

31. Fig. Dorsal closure of the embryo.

32. Formation of the Gut Cells at the anterior and posterior ends of the embryo migrate inward to form the foregut, hindgut, and midgut. – –Foregut and hindgut: raise from ectoderm cells. – –Midgut: from the endoderm (some of the invaginating epithelia) The endodermal cells form sheets that enclose the yolk in a tube that creates the midgut epithelium. Vitellophages contained within the yolk that are integrated into the endoderm to form the definitive midgut epithelium.

33. Fig. Formation of the foregut and hindgut from ectodermal invaginations and the development of endoderm that forms the midgut.

34. Formation of the Nervous System The nervous system arises from ectodermal cells in the ventral region of the germ band. Proliferation of the neuroblasts in portions of the embryonic ectoderm produces a neural groove and neural ridges. Three groups of neuroblasts that proliferate in the anterior region ultimately produce the procerebrum, deutocerebrum, and tritocerebrum of the brain. Those in other segments give rise to the subsesophageal and abdominal ganglia.

35. Fig. Formation of the ventral nerve cord from neuroblasts.

36. Formation of Internal Organs Mesodermal tissue gives rise to most of the internal organs of the insect. – –Splanchic mesoderm: forms the visceral muscles – –Somatic mesoderm: gives rise to the skeletal muscles

37. Formation of the Reproductive System The reproductive system is constructed around the pole cells. The pole cells become the germ cells of the future adult. The genital ridge of mesoderm that forms the reproductive organs, germaria of the ovarioles and the follicles of the testes. Mesoderm also gives rise to the lateral oviducts and vasa deferentia. Invaginations of the ectoderm form the median oviduct and ejaculatory duct.

38. Fig. Cell lineages and derivation of tissues in the mature insect.

39. Endocrinology of Embryonic Development Hormones required during embryogenesis are packaged into the egg largely as inactive hormone conjugates and released from the conjugates as active hormones when they are required. – –Ecdysteroids: involved in embryonic molting. – –Juvenile hormone: less known about its contributions during embryogenesis – –Diapause hormone in Bombyx mori eggs

41. Two Way to Create a Morphogen Gradient

42. The Origins of the Drosophila Body Segments During Embryonic Development

43. A Drosophila oocyte in Its Follicle

44. The Organization of the Four Egg- Polarity Gradient Systems

45. Morphogen Gradients Patterning the Dorsoventral Axis of the Embryo

46. Examples of the Phenotypes of Mutations Affecting the Three Types of Segmentation

47. The Regulatory Hierarchy of Egg-Polarity, Gap Segmentation and Homeotic Selector