InnerNature: Sex — The key to diversity

By Vidya Rajan, Columnist, The Times

In his book “Evolution: The Triumph of an Idea,” Carl Zimmer concedes: “Sex is not only unnecessary, but it ought to be a recipe for evolutionary disaster. For one thing, it is an inefficient way to reproduce…And sex carries other costs as well…By all rights, any group of animals that evolves sexual reproduction should be promptly outcompeted by nonsexual ones. And yet sex reigns… Why is sex a success, despite all its disadvantages?”

Sex does present a conundrum. However, considered through an evolutionary lens, sex makes complete sense as a way to shake things up in the gene pool, so that new permutations and combinations of genes, and thus new individuals carrying those new combinations can exist. According to evolutionary theory, the best combinations of genes will produce the most competitive individuals a product. Evolutionary success is measured only in reproductive terms, because competitive and successful individuals will pass on their gene collections which will undergo mixing and selection ad infinitum. This implies that the current “pool” of genes are those that were present in a long chain of previously reproductively successful owners, but in different gene company. Barring identical twins, the combinations of genes present in your body have never existed before, and never will again. Sexual reproduction merely provides a mechanism to shuffle the genes in the gene pool and redistribute them such that new combinations emerge, some of which are successful and some not. This has existential consequences with some combinations which survive long enough to reproduce, and others that don’t – successful combinations of genes will survive for future mixing; if they do not reproduce, end of story.

A cell division process called meiosis shuffles genes in a cell when making gametes – eggs or sperm. This it does by physically breaking and rejoining chromosomes, thereby scrambling gene combinations. Further, whole chromosomes pick up and move into sperm or eggs in combinations with chromosomes from another parent whose genes were also scrambled. This results in a chromosome with gene for mom’s gluten intolerance and dad’s diabetes combining in a cell with another chromosome with a gene for dad’s great teeth and mom’s sparkling eyes. This is oversimplifying a lot, but the sperm or egg now contains genes, good and bad, that initially came from separate ancestors. An egg with one set of genes, half of the full complement, will combine with a sperm from a different person with another set of genes providing the other half of the complement, and a new baby with unique combinations of genes that never before have been seen, will be born. This baby will have two copies of every gene that survived, from two parents, four grandparents, and eight great-grandparents and so on in any permutation and combination. Some babies’ combinations will successfully navigate all the obstacles of life to have babies of their own, and those genes that pass to new generations will mix and match as well.

Therefore, sex exists to provide variation in offspring.

But how do organisms manifest sex? Let’s go back in time and look at some of the earliest living things and follow them to examine the prediction of “endless forms most beautiful” with regard to sex. One of the fun things about biology is comparing across different kingdoms of life. I want to do this because many people don’t realize humdrum things like mosses and fungi have sex lives. Well, they do.

Primitive fungi, some algae, and animal-like protozoa produce gametes that are isogamous, that is indistinguishable from each other externally, but they are not identical. They have biochemical differences that lead them to find and fuse with a genetically dissimilar gamete. These biochemical differences are tantamount to sexes, and some fungi can have as many as 36,000 mating types! The problem is that these gametes must both be smallish to travel significant distances to find a partner. The embryo that results from fusion of two smallish gametes will have limited energy stores and few resources. The margin for error is small – if the embryo cannot find energy to grow immediately and sustain itself, it will die. Isogamy is not for the weak hearted.

To give the embryo some additional resources for its start in life, a clever strategy called anisogamy was deployed. One set of the gametes enlarges, and accrues resources and energy. By convention this gamete is called an egg, and the parent the female. Over evolutionary time, this egg got bigger and bigger, stockpiling energy and other resources. It eventually became large and heavy, although few in number. The male gamete, by convention called the sperm, shed all its resources except for the chromosomes it was bringing and a little bit of energy to travel. The male parent invests most of its energy into making a lot of sperm, each with just a little bit of energy to propel it egg-wards. This is the solution for sex used by animals, most plants, and some fungi and protists.

How do sperm get to the egg? Sperm released into water for external fertilization have to cope with currents, dilution, and distance. Sperm have a flagellum, a tail that works like a rudder and propellor, with chemical sensors on the front to sniff out the egg. Water is a good medium because it prevents drying. To overcome distance and dilution problems, many water-dwelling animals like coral anemones, fish, and frogs release their gametes simultaneously to make it easier for them to find each other and fertilize successfully. In wetland environments, plants like mosses, ferns and some trees of ancient lineage have flagellated sperms which swim to the egg. In fact, the largest sperm in the world, just about visible to the naked eye, is a cycad sperm. It is as large as a grain of salt!

On dry land, matters are different and sperm have evolved to cope with the problem of drying out. The pollen of plants is sperm placed inside a waterproof coating. For many people, the mass spewing of pollen in the spring causes allergies and sneezing. Airborne pollen is thrown out in vast quantities by many pines and grasses because of the hit-or-miss nature of its wind-borne pollination. On the other hand, flowers beckon to pollinators to transfer their pollen between them, giving them  nectar and a little pollen to eat as reward. Then they don’t need to make as much pollen. [Note: Isn’t it funny to think that when you smell a bunch of flowers, you are essentially doing the same thing your dog does when it makes a beeline for a stranger’s crotch.] The pollen coat around the sperm is so durable that it is used palentologically, archaeologically, and forensically. Beekeepers understand the value of palyology, which is used for identifying pollen in honey to establish nectar provenance. Among animals, arachnids encase their sperm in “packets” that the female picks up and internalizes to fertilize her eggs. Drone honeybees deliver their spermatheca inside the queen bee by tearing it out of their bodies and falling, eviscerated, to their deaths. Many male land animals, including humans, deliver their sperm periodically rather than all at once, transferring it into the female’s body using specialized apparatus, and sometimes even forge relationships to nurture and protect the young until they can fend for themselves.

The female’s role in egg production took the egg from being small and unremarkable to huge and very remarkable. Consider a chicken’s egg. As a stand-alone source of energy, it is so valuable that many other animals steal the resources for themselves, killing the embryo in the process. The moral is that even good strategies can backfire. Primitive mammals such as the echidna and duck-billed platypus lay eggs but nourish the offspring with milk. More advanced mammals hatch the eggs within their bodies, protecting and nourishing the growing embryo until they are partly or fully mature. Marsupials give birth to relatively undeveloped fetuses that grow outside the mother’s body, whereas placental mammals protect and nourish the embryo inside their body until it is quite well developed.

Retention of sperm, fertilization of an egg, and the development timelines of an embryo are remarkably varied. Plant eggs – what we call seeds – show dormancy, going into suspended animation, sometimes for decades. Queen honeybees can retain sperm inside them for up to 5 years, releasing sperm at will to fertilize eggs or not, thereby producing workers or drones, respectively. The panda, rodents, bears, and roe deer can maintain a fertilized egg in limbo –  called embryonic diapause – by preventing its implantation in the uterus until conditions are appropriate.

Endless forms most beautiful and most wonderful it certainly is, but sexual reproduction also provides the variation that sustains competition and ensures there is enough genetic diversity in the population to overcome survival challenges for the species now and in the future. The true promise of sexual reproduction is that your children will not be a tiny version of you.

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