lecture 8: seasonal breeding

Question 1

What are patterns of breeding cycle?

Answer 1

many patterns:

• continuous
• continuous with seasonal variation
• extended period of breeding
• very brief breeding season
• variation within species depending on environment

Question 2

Why do we have environmental control of breeding cycles?

Answer 2

• environmental variation affects survival
• powerful selection pressure to time breeding to maximise reproductive success
• ultimate factors provide the evolutionary selection pressure and include food, temperature, rainfall, humidity
• because in many environments the availability of food, temperature etc vary seasonally, and many species breed on an annual cycle cued by photoperiod as a proximate factor
• not all species breed seasonally
  
  • continuous
  • opportunistic cued by environmental factors
• male – female differences in patterns

Question 3

What is an example of food as a proximate factor?

Answer 3

• nutritional influences on reproduction
  
  • calorie balance
  • hormones or other factors in feed

Question 4

What is an example of hormones and other factors in feed affecting timing of breeding?

Answer 4

• reproduction response of microtus to wheatgrass supplements
• date : treatment : % pregnant
• 23-25 jan : no grass : 0
• 15 - 18 feb: no grass : 0
• 15 - 18 feb: grass supplement : 100
• 12 - 14 mar : no grass : 5
• 10 - 12 april : no grass : 96

Question 5

How can rainfall be used as a cue?

Answer 5

• correlation among annual pattern of rainfall, number of insects available and percentage of sheath-tailed bats lactating in coastal kenya

Question 6

What can influence flexibility of breeding patterns?

Answer 6

• flexibility in breeding patterns depending on location
• alternate cues
• alternate use of cues
• rapid selection for variants that maximuse success

Question 7

What is a photoperiod?

Answer 7

• correlates with seasonal changes in weather, food etc
• most obvious changes in temperate environments
• photoperiod changes with latitude
  
  • little photoperiod change in equatorial region
  • extreme photoperiod change in polar regions
• absolute daylength vs change in daylength
• allows long-term synchronisation e.g. autumn mating for spring births

Question 8

What does photoperiodic control require?

Answer 8

• photoreceptor (clock)
• neural pathway linking clock to neuroendocrine pathways
• endocrine response of hypothalamo-pituitary-gonadal axis
• variation between species
  
  • short day breeders
  • long day breeders

Question 9

Why have seasonal breeding?

Answer 9

• climate and food availability = survival
• ensure offspring born best time
• more pronounced away from tropics (greater seasonal variation)
• ultimate factors: temperature, rainfall, food availability
• proximal factors or predictors: daylength
• regulating time of conception:
  
  • short gestation spp: spring e.g. hamster, birds
  • long gestation spp: autumn, e.g. sheep, deer
  • variable gestation spp (delayed implantation/embryonic diapause) e.g. kangaroo, mustelids, seals
• alignment with season crucial for species survival

Question 10

What is the breeding pattern in Soay sheep?

Answer 10

• primitive sheep
• reflect the sorts of breeding in sheep before they became domesticated
• become anoestrus following breeding season
• very tightly coordinated: march through to may
• mortality higher in those born later
• very strong selective pressure

Question 11

What are the circannual rhythms in the Soay ram?

Answer 11

• high levels of prolactin in winter
• FSH levels increasing in advance of rut
• sequence of seasonal reactivation similar to puberty
• low plasma FSH/LH in spring, LH surg, FSH risin in summer, lower than summer but higher than basal in autumn (pulsatile), testis size increased in autumn, higher pulse rhythm of LH
• LH pulses correspond with testosterone, has less frequent pulses than FSH
• other circannual rhythms
  
  • moulting, antlers, food intake, weight gain, seasonal breeding

Question 12

What are circadian rhythms in rats?

Answer 12

cyclical activities in the female rat

• nocturnal activity cycle – a 24 hour cycle
• production of oocyte – a 4 - 5 day cycle
  
  • LH surge (2-4 pm), 5 - 7 h before darkness
  • ovulation (2-4 am), during active period
• other circadian rhythms: temperature, melatonin, prolactin, growth hormone, corticosterone, sleep-wake, etc

Question 13

What is the pineal gland?

Answer 13

• endocrine gland in roof of brain
• photoreceptive in lower vertebrates
• receives photic input via suprachiasmatic nucleus and superior cervical ganglion
• makes the indole hormone melatonin in dark

Question 14

what is the photo-neuro-endocrine pathway?

Answer 14

seasonality is disturbed by:

• blinding
• lesions of the SCN
• ablation of the SCG
• pinealectomy
• photoperiod changed and continuous light or dark but usually takes a few days for endogenous rhythm to adjust
• long-lasting melatonin implants

Question 15

What is the control of melatonin secretion?

Answer 15

• Tryptophan
  
  • tryptophan-5-hydroxylase
• Serotonin
  
  • N-acetyl-transferase (NAT)
• N-Acetyl-serotonin
  
  • hydroxynindole-o-methyl-transferase (HIOMT)
• melatonin

also input on NAT:

• nocturnal stimulation
• NA - sympathetic nerves
• beta receptor
• c-AMP
• promotes NAT

Question 16

What are the effects of photoperiod and melatonin on LH in ewes?

Answer 16

Question 17

What were the melatonin profiles in Soay rams moved to continuous dark?

Answer 17

• endogenous melatonin cycle persists in dark but is normally entrained by light-dark cycle

Question 18

What is the retino-hypothalamic-pineal tracT?

Answer 18

• more complicated
• where in the system is the endogenous system sat
• just remove light: still a 24 hour cycle
• take out superchiasmatic nucleus and flat line
• SCG - flat line
• dark - cycle
• light - flat line

Question 19

What is the hormone of darkness?

Answer 19

• melatonin
• melatonin secreted at night
• synthesised in the pineal gland and released
• reflects the length of the night
• changing melatonin profile alters GnRH secretion

Question 20

What is the distribution of melatonin binding areas?

Answer 20

• MT receptors in medial basal hypothalamus and pars tuberalis
• MT affects electrical activity GnRH neurones
• MT affects synthesis of neurotransmitters

Question 21

What is prolactin secretion and the pelage/moult cycle?

Answer 21

• seasonal cycle in prolactin secretion in syrian hamster and soay shep
• SD = low Prl
• LD = high Prl
• photoinduction = response to change from SD to LD or vice versa
• refractoriness = inhibitory or stimulatory effects of photoperiod wear off

Question 22

What is the biological clock?

Answer 22

• self sustaining biological clock or oscillator in SCN
• constant light or dark free - runs c. 24 h
• located in suprachiasmatic nucleus
• entrained by photic stimuli via retinogypothalamic tract (Feeds photoperiod info to clock in SCN)
• lesion to tract (SCN to SCG to pineal) disrupts circadian rhythms
• rhythm expressed by melatonin secretion
• melatonin receptor found in MBH and PT
• circannual cycles in prolactin associated with pelage
• pars tuberalis has calendar cells

Question 23

So how do we get control of seasonal breeding?

Answer 23

• CNS
  
  • integrates environmental cues like like, olfactory stimuli, temperature…
  • neural signals
• pineal gland
  
  • transducer of photic information from retina
  • involved in time perception via clock genes
  • makes melatonin in dark
  • melatonin
• hypothalamus
  
  • GnRH release modulated by melatonin (via MBH?)
  • regulation of steroid feedback
  • pars tuberalis produces prolactin
  • GnRH
• Anterior pituitary
  
  • tuberalin from pars tuberalis generate prolactin
  • GnRH regulates LH and FSH secretion
  • LH and FSH
• Gonads