W3L2 Tues Seasonal breeding Flashcards
Pattern of breeding cycle
Many patterns
* continuous
* continuous with seasonal variation
* extended period of breeding
* very brief breeding season
* variation within species depending on environment
Environmental control of breeding cycles
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
* These vary seasonally, and many species breed on an annual cycle- need to use a proximate factor e.g. photoperiod
* Not all species breed seasonally
– continuous
– opportunistic cued by environmental factors
* Male - female differences in patterns
Flexibility in breeding patterns depending on what
- Alternate cues
- Alternate use of cues
- Rapid selection for gene variants that maximise success
Photoperiod
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 for reliable long-term synchronisation
e.g. autumn mating for spring births
Seasonal breeding patterns requierment (photoperiodic control)
Photoperiodic control requires 3 components
1. Photoreceptor (clock)
2. Neural pathway linking clock to neuroendocrine pathways
3. Endocrine response of hypothalamo-pituitary-gonadal axis
Variation between species
– short day breeders
– long day breeders
Soay sheep breeding cycle
Found on Isle of St Kilda (Arctic Circle) = extreme changes in photoperiod
§ Mate in autumn, birth in spring (5 months ) + anoestrus in spring to summer
§ Period of birth very short but 1st winter mortality much greater in lambs born later
Ø Lambs born earlier able to grow + had an advantage in harsh Scottish winter
§ Circannual rhythms in Soay rams:
Ø Spring (long daylength): ↑prolactin, low FSH/LH, low testosterone
Ø Summer: ↑FSH/LH, onset of spermatogenesis
Ø Autumn (short daylength): ↑testosterone, testes large (sperm production)
Circadian rhythm and rat
Female rat: 4-5 day oestrous cycle that depends on photoperiod
Ø LH surge at 2-4pm (5-7h before darkness) + ovulation at 2-4am (during active period)
Ø Neural signal drives synchronisation of LH surge
Ø Reversing light cycle can shift time of LH surge by 12 hours
The Pineal gland
endocrine gland in roof of brain that produces melatonin in the dark
Ø Evolutionary origin: a photoreceptive organ in lower vertebrates
Ø Receives photic input via suprachiasmatic nucleus (SCN) + transported to brain via superior cervical ganglion (SCG)
§ Suprachiasmatic nucleus (SCN): internal body clock regulating most circadian rhythms
Ø Produces melatonin at night + Can keep melatonin production going- even if time ‘lost’ = jet lag
The photo-neuro-endocrine pathway
§ Eye → retino-hypothalamic tract → SupraChiastimaticNucleus → spinal cord → Superior Cervical Ganglion → pineal gland
Ø SCG sympathetically innervates the pineal gland
Ø Pineal releases melatonin → regulates GnRH → LH/FSH → gonads
§ Seasonality disturbed by: blinding, lesions of SCN, ablation of SCG, pinealectomy, photoperiod changes + continuous light/dark, long-lasting melatonin implants
Control of melatonin secretion
Neuronal signals turns on enzyme in the rate-limiting step of melatonin production
melanin production
Nocturnal stimulation of sympathetic nerves→stimulate beta-adrenergic receptors on pineal gland→ cAMPrelease
-cAMP lead to activation of N-acetlytranferase (NAT) in conversion of tryptophan to melatonin
-In pineal gland: tryptophan -> serotonin -> N-acetlyase serotonin -> melatonin
-Longer night = longer melatonin production > higher LH production by altering GnRH secretion. Melatonin receptor in the medial basal hyphothalamus
Prolactin (PRL)
Secreted by anterior pituitary lactotrophs, placental tissues, many other tissues
* >300 functions in vertebrate systems (seasonal breeding, metabolism, immune etc)
* Main reproductive actions:
– Male – generally high levels turn off GnRH and testosterone production. However, low levels aid LH by ↑ LHR on Leydig cells, accessory organs
– Female – generally promoted by oestrogen and aids LH by ↑ LHR in follicle, corpus luteum, mammary gland (lactation!), amnion
* Inhibited by prolactin inhibitory factor (PIF; dopamine) from the arcuate nucleus
Prolactin secretion and the pelage/moult cycle
Seasonal cycle in prolactin (PRL) secretion in Syrian hamster and Soay sheep Moult in summer (Long-day)
* Short Day (SD) = Low PRL
* Long Day (LD) = High PRL
Photoinduction = response to change from SD to LD or vice versa
Refractoriness = inhibitory or stimulatory effects of photoperiod wear off
All photoperiodic species show high prolactin under long days
The Biological Clock
Self-sustaining biological clock or oscillator in suprachiasmatic nucleus (SCN)
* Constant light or dark free - runs about 24 h cycle
* Entrained by photic stimuli via retino-hypothalamic tract (feeds photoperiod info to clock in SCN)
* Lesion to tract (SCNSCGpineal) disrupts circadian rhythms
* Rhythm expressed by melatonin secretion
* Melatonin receptor found in MBH and PT
* Circannual cycles in prolactin associated with pelage
Control of Seasonal Breeding
CNS integrates environmental cues (light, olfactory stimuli, temperature) → neural signals → pineal gland (transducer of photic info form retina, makes melatonin in dark) → melatonin → hypothalamus (GnRH release modulated by melatonin) → GnRH → anterior pituitary (pars tuberalis secretes prolactin, GnRH regulates LH + FSH) → LH/FSH → gonads
