In the Life Science's lab at the RSM, we have a few freezers which are used to keep samples and specimens frozen until they are ready to be processed. Often, the purpose of the cold treatment is to keep tissues from decomposing, but also, and often more importantly from a research perspective, low temperatures keep the DNA (i.e., genetic material) within the cells "alive" for future genetic study. Ultra-low temperatures (i.e., -80°C) are routinely used in medical facilities, science labs, and other facilities around the world for the preservation of cellular material. One instance of relevance to our species is long-term sperm storage through cryopreservation, which can be done to preserve the option to father children prior to undergoing a vasectomy, cancer treatment, etc. which will/may lead to infertility. The process involves a combination of cold temperatures, and cryoprotectants (i.e., antifreeze mixtures of sugars and alcohols). Viability under these conditions remains high for over 20 years, and in the future, I suspect for much, much longer. Thus, the possibility for plotting what seems like a great science fiction story line is possible. The science though, is interesting in itself!
For an interesting twist on this, consider the highly social honey bee, which has evolved its own way of keeping sperm viable for long periods of time. Honey bees, as we know, live in perennial colonies that can exist (theoretically) forever. However, this involves continuous replacement of workers within the colony, and also new queen production and swarming to establish new colonies. Thus, the colony can continue to exist, though the individual occupants are routinely replaced. The new queen, before taking over the colony inherited from her mother (who has left with approximately half the workers to start a new colony) first leaves the hive for a mating flight. The "honeymoon" can last for some time, and although we know very little about the mating rituals of honey bees, we do know from genetic studies of sperm within the queen and from in-hive worker relatedness studies, that she mates several times during this flight, and returns to the colony, never to mate again. I will say little on why she mates multiple times, as there are many theories, but from the perspective of colony longevity, it is beneficial to have diverse genetic makeup. It needs to be stressed that this queen will be laying eggs for the next 5 years or longer (more on fertilization in bees in another blog). This gets me back to the point of this blog; how does the queen keep the male[s] contribution to the longevity of the colony alive, for years, at temperatures held at pretty much a consistent 30°C?
It is an interesting question, and in most vertebrate animals, sperm remains alive and active in the female for only a few days to weeks after mating. This is true for most mammals, though hibernation allows this to be extended to months in some bats. The exceptions to the rule for other vertebrates include some species of reptile, in which sperm survival can remain high for months to years. However, in the animal kingdom, the record holders are insects, in particular honey bees and ants: sperm from the initial mating events can remain viable and functional within the queen for decades! What allows sperm to live so long in these groups? The short-term livelihood of sperm, like in most animals, is maintained by the fluids transferred along with sperm from the male. These fluids contain proteins and sugars that nourish the sperm for the short-term while in the female. Interestingly, sperm survive much better when they stay in their "own" seminal fluid rather than in that contributed by another male, and the queen secrets chemicals that diminish these male plots to succeed in mating. This still does not answer the question of long term survival, and at present, we are not sure of all the details. We do now know that for extended storage, the nutrition for the sperm comes from the queen. After mating, the sperm is held in specialized storage organs (called spermathecae) and released in small quantities for each fertilization event. Secretory cells within the storage organ play a huge role in nourishing the sperm, but much remains to be learned about how this occurs, and what substances are involved. From a pure research perspective, it will continue to be of great interest to determine what specific molecules are involved (there are some studies already), and to discover what are the underlying mechanisms promoting sperm survival. These findings will thus have great implications in medicine, as discussed above, and for life sciences in general; keeping individual cells alive and healthy for long periods of time without cold temperatures has great implications for conservation of species, including our own.