Biology · MU Sofia

73. Phylogeny and development of reproductive system..

Traces how the reproductive system has evolved across vertebrates, covering gonads, sperm production, egg development, female genital tract variation, fertilisation and the three reproductive strategies. It then explains embryonic origins of the gonads and genital ducts, the role of the Sry gene, and common malformations such as cryptorchidism, hypospadias and uterine anomalies. Recap questions and four mnemonics, including ones for reproductive strategies and uterine types, are included.

Inside: 5 sections · 16 flashcards · 9 min

Topic 74 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 5 sections, about 1,213 words (9 min of reading), with 16 flashcards.

In this note

  1. Phylogeny and development of reproductive system
  2. Phylogeny and Development of the Reproductive System
  3. Phylogeny: Evolutionary Background of the Reproductive System
  4. Development: How the Reproductive System Forms in the Embryo
  5. Mechanisms of evolution. Anagenesis.

Phylogeny and Development of the Reproductive System

The reproductive system, or Systema genitalis, is a complex and vital organ system that has evolved over time to ensure the continuation of species. Its structure and development are tightly linked to evolutionary history and embryonic development.

Phylogeny: Evolutionary Background of the Reproductive System

Gonads (Ovaries & Testes):

  • In most vertebrates, females produce either many small or few large eggs packed with yolk, making their ovaries generally larger than testes.
  • However, mammals are different — their eggs are tiny and lack yolk due to the evolution of the placenta. As a result, their gonads are more similar in size.

Sperm Production (Spermatogenesis):

  • In anamniotes (like fish and amphibians), sperm develops in cysts of Sertoli cells. All sperm in a cyst mature together, then the cyst bursts to release them.
  • In amniotes (like reptiles, birds, and mammals), sperm develop in seminiferous tubules, allowing for continuous and high-output sperm production.

Oocyte (Egg) Development in Non-Mammals:

  • Non-mammalian females grow giant oocytes to store enough nutrients.
  • Their chromosomes form lampbrush loops to allow intense transcription during growth.
  • The liver plays a major role by producing vitellogenin, a yolk protein precursor, which is delivered via the blood to the growing egg.
  • Therian mammals (marsupials and placentals) no longer use vitellogenin, having evolved alternative strategies for nourishment via the placenta.

Female Genital Tract:

  • Birds develop only the left ovary and oviduct—this helps with flight by reducing weight.
  • In placental mammals, the uterus may be:
  • Duplex (fully separated),
  • Simplex (fully fused, like in humans),
  • Or a mix, with partial fusion.
  • Monotremes, like the platypus, are egg-layers and have reptile-like reproductive tracts.

Fertilization & Egg Laying:

  • In species with internal fertilization, this occurs in the upper part of the oviduct.
  • In oviparous amniotes, eggs pass through the oviduct, receiving layers like egg white (albumin) and a shell before being laid.

Reproductive Strategies:

Vertebrates have three modes:

  1. 1. Oviparous – Lays eggs (e.g., many fish, reptiles).
  2. 2. Ovoviviparous – Eggs hatch inside the mother; young are born live.
  3. 3. Viviparous – Direct nourishment via placenta; young are born live (e.g., mammals).

Copulatory Organs:

  • These are adaptations for internal fertilization:
  • Claspers in cartilaginous fish (e.g., sharks).
  • Gonopodium in some bony fish.
  • Hemipenes in reptiles (e.g., snakes).
  • Penis in mammals—sometimes with a baculum (penis bone).
  • Amphibians, interestingly, do not have copulatory organs.

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