BIOL-MARI 3370 Lect 17 F2024 (1) PDF

Sexual Reproduction Sockeye salmon Chpt 17  Sexual Reproduction: as defined by the ovarian cycles of females Most female animals are spontaneous ovulators, meaning that ovulation results from an endogenous process in the female,...

Sexual Reproduction Sockeye salmon Chpt 17  Sexual Reproduction: as defined by the ovarian cycles of females Most female animals are spontaneous ovulators, meaning that ovulation results from an endogenous process in the female, usually synchronized to some external process (e.g. seasonality) or to an endogenous clock. Whales, fish, moose, humans, etc. In contrast, induced ovulators ovulate in direct response to sexual stimulation, irrespective of calendar date or environmental cues (e.g. rabbits, hares).  Timing of Reproduction Amongst spontaneous ovulators, many females are seasonal breeders. Timing of breeding is linked to predictable environmental cues. Change in photoperiod is the most predictable cue. Arctic tern with chick  June The Analemma Noon-time, as viewed weekly from Edmonton. The suparchiasmatic nucleus tracks daily light levels and translates this into a chemical cue via the pineal gland (melatonin). December This cue is entrained to establish a circannual rhythm. … Seasonality. Physiological relationship with Photoperiod White-crowned sparrow Plasma Gonadotropin releasing hormone Testicular mass Winter Spring Summer Fall Winter Photosensitive Photorefractory  Timing of Reproduction in seasonal breeders Integration of seasonal photoperiodic cues trigger the initiation of reproduction – while other factors (temperature, food abundance, etc) can fine-tune the actual timing of breeding. Simultaneously, many (but certainly not all) animals will migrate from winter non-breeding areas to summer breeding areas. All must prepare physiologically for migration and breeding. HPG Axis: Hypothalamus-Pituitary-Gonad Pleiotropic effects of sex steroids on Hypothalamus BEHAVIOUR Gonadotropin releasing hormone Pituitary Migration Courtship Follicle stimulating hormone Luteinizing hormone Breeding activity Ovaries, Testes Progesterone, Testosterone, Estradiol GAMETES Reproductive physiology - Integration of photoperiodic cues Suprachiasmatic Pineal Gland nucleus Melatonin vole Optic optic nerves chiasma Hypothalamus Photoperiods are calculated by the suprachiasmatic nucleus. Gonadotropin This regulates pineal secretion of melatonin, which then regulates releasing hormone hypothalamic gonadotropin releasing hormone secretion. Recall that pineal release of melatonin is inverse to daylength. So Pituitary in spring when days get longer, the pineal releases less melatonin (i.e. shorter nights). When melatonin drops below some threshold Follicle stimulating hormone Luteinizing hormone level in spring it releases the hypothalamus from inhibition and permits gonadotropin releasing hormone secretion. Ovaries The pituitary then releases the gonadotropins: Follicle Testes Stimulating Hormone and Luteinizing Hormone, which simulates the production of 3 sex steroids in the gonads. Progesterone, Sex steroids needed for production of gametes. Testosterone, Estradiol Reproductive physiology - Integration of photoperiodic cues Suprachiasmatic nucleus X Pineal Gland vole X optic nerves Optic chiasma Hypothalamus “Blinding” halts the HPG axis Pituitary If experimentally blinded, some seasonally breeding mammals won’t up-regulate their HPG axes. Ovaries Testes No reproductive development occurs. Reproductive physiology - Integration of photoperiodic cues X Suprachiasmatic Pineal Gland Encephalic nucleus photoreceptors (Melatonin) X Optic Zebra chiasma finch optic nerves Hypothalamus Gonadotropin releasing hormone Birds are different! Eyes, pineal gland, and Pituitary melatonin are not required for regulation of HGP axis. Follicle stimulating hormone Luteining hormone Presence of photoreceptors in the brain detect Ovaries photoperiod cues, which can regulate Testes hypothalamic gonadotropin releasing hormone secretion directly. Progesterone (Saldanha et al. 2001). Testosterone, Estradiol Reptiles and amphibians have a parietal eye, which serves an important role in circadian rhythmicity of ectotherms, but is absent in endothermic birds and mammal. Juvenile bullfrog (Lithobates catesbeianus)  Reproductive physiology - INDUCED OVULATORS afferent Brainstem neurons neurons Cervical stimulation Norepinepherine during sex Hypothalamus Hare Gonadotropin In only a few animals - hares, rabbits, camels, releasing hormone alpacas, llamas - physical stimulation of stretch Pituitary receptors on the cervix sends impulses via afferent neurons to the brainstem. Luteinizing hormone This stimulates Norepinepherine release, Ovaries which simulates an HPG cascade to bring about a surge of Luteinizing hormones, leading to ovulation. OVULATION  Timing of Reproduction Successive steps in reproduction can be… Rigidly Linked = mating, fertilization, and embryonic development all occur without interruption or delay (most animals). Decoupled = animals have flexibility in the coordination of these events to match some environmental or abiotic condition. Blue crab  Decoupled or Delayed Reproduction Tactic 1: Sperm storage Blue crab – females can mate only when molting, but can store sperm and fertilize eggs when she deems conditions to be best. Breeds throughout the year. Tactic 2: Embryonic diapause Programmed slow development In placental mammals, sometimes called delayed Implantation. Two forms - Obligate or Facultative  Facultative Embryonic Diapause Requires 2 months at < 5°C Lay eggs D I A P A U S E Hatch Spring Summer Autumn Winter Spring Silk moths: Reproduce in summer (parts of Asia). Diapause hormone secreted into eggs by mother. After fertilization, this suspends embryonic development at the gastrula stage. Eggs are then laid in autumn. Temperature sensitive process – only after eggs have experienced 2 months of winter temperatures