What is the smallest or simplest/most primitive organism that reproduces sexually?
Are there any single-celled organisms that need to perform some sort of exchange (obviously not sperm/egg) before they can reproduce? (I don’t think this would be ‘proper’ sexual reproduction, would it?)
Not the smallest, but the simplest may be sea sponges. These are often considered the simplest animal, and can reproduce both sexually and asexually. They are hermaphrodites, and sexual reproduction takes place by a release of sperm into the water and the hope that the sperm finds its way over to another sponge of the same species. Not as much fun as our way, but certainly both easier and cheaper.
As for smallest, it might depend on your definitions. Bacteria can exchange genetic information, which is then passed on to their offspring through asexual division. One result of this ability is the increase in antibiotic resistance in bacteria; one resistant bacterium can pass that gene not only to its descendants but to other bacteria and their descendants.
The answer depends on how you define “sexual reproduction”. If we take the loosest definition of it being some sort of genetic exchange and recombination, then even single-celled bacteria routinely exchange DNA in a process called conjugation. Of course, this is not required for reproduction, since bacteria can reproduce just fine by simply dividing asexually.
Paul Williams?
According to my ex … me.
-Syko
“My cat’s breath smells like cat food.” - Ralph Wiggum
Yes there are and yes it’s proper sexual reproduction.
Diatoms are unicellular algae that produce two silicon shells around themselves like little bitty clam shells. They can reproduce asexually by cell division, with each daughter cell getting half of the parent shell and growing another half. However amongst many species the arrangement of the two halves is such that one half is smaller than the other, and the parent shell always forms the large half. This means that after a few divisions the shell becomes too small to contain a functioning organism. They then undergo sexual reproduction, producing either 4 motile sperm cells or one egg. After fertilisation the zygote expands to full size and the whole mess starts again.
So that means that these unicellular organisms need to sexually reproduce every few generations to survive so I guess they qualify as the winners.
If you’re looking for organisms that can only reproduce sexually then I would hazard a guess that some of the microscopic roundworms would be the winners since some (though not all) are only capable of sexual reproduction. Sponges don’t qualify since all sponges can reproduce asexually.
according to wikipedia, " bacterial transformation can be regarded as a form of sex in bacteria."
on the other hand, bacteria conjugation is NOT sexual reproduction
Rotifers are one example of a small and simple organism that typically (but not exclusively) reproduces sexually. See the Wikipedia entry for some of the odd variations involved. (For example, and entire class of rotifers, the bdelloids, reproduce only parthenogenetically – all bdelloid rotifers are physiologically female. Nonetheless, they have diversified into over 300 species.)
Also, a lot of unicellular parasites have an asexual stage AND a sexual stage. The parasite that causes malaria, for example, can reproduce asexually through multiple fission (schizogony) but also sexually when the gametocytes bind to form a zygote. This parasite, Plasmodium, reproduces bymultiple fission inside human cells but then undergoes gametogenesis to produce male and female gametocytes (microgametocytes and macrogametocytes). Then when these gametocytes enter the Anopheles mosquito (when it feeds on infected blood), the microgametocyte enters the macrogametocyte and together they form a zygote, which thenmultiplies meitotically in a process known as sporogony, which produces infective sporozoites. These sporozites enter and infect another human host when the infected mosquto bites him/her. Then these sporozoites enter a liver cell and multiply, then its products enter blood cells and multiply and then this person can be diagnosed with malaria.
So, to summarize, unicellular organisms (other than bacteria) can reproduce sexually if they undergo gametogenesis and some stage in their life cycle.