Convergent evolution is truly a simple yet beautiful theory: if different living creatures are facing similar stress from their respective environmental conditions, sometimes they would adopt the same strategy to overcome those obstacles. For example, both bats and dolphins have undergone the same genetic mutations to use echolocation for hunting, despite one animal living on the land and the other in the sea. Or just look at how the birds, bees and bats have all undergone different evolutionary steps to gain the ability of flight.
Qazho, the bees of the moon
The second biggest moon of Kepler 452-b, which was yet to be discoverable by humans, was roughly 10% smaller than Mars by size, but had similar composition as the Earth, meaning that it had an oxygen-rich atmosphere, ginormous waterbodies like the oceans, and continent-sized landmasses that were rich in useable nitrogen and phosphorus.
There was one particular species on the moon that looked unbelievably similar to the bees on Earth: the Qazho. Their appendages had greatly reduced in number down to three pairs. Their body color was turquoise and with stripes of fluffy yellow hair. They had two pairs of wings and were fantastic pilots in the air. They had inherited two sets of visual sensory organs, including three ocelli on the top of the head (for acute reception and processing of micro-weather information) and two fairly large compound eyes on each side of the head (for high-resolution vision when they were hovering, at rest, or flying at low speed). They thrived in the cold, polar regions of the moon where they were the only specialist scavenger of the highly toxic Qowaris’ faeces that were constantly washed up in abundance from the “oceans”. The compound benefit of their special niches and harsh habitat was that they were free of predators and competitors on the land: they were the king of the land.
But what was even more impressive, which was also the deciding factor that set them apart them from bees, was that they possessed much higher cognitive intelligence than humans.
Well – this is only half the truth.
The zombie fungus
The thing was, Qazho was in a mutualistic relationship with a fungus-like creature, Qciz, that was originally a deadly parasite to the Qazho. At the very beginning, multiple animals were in a race to overcome the ocean-to-land barrier. At that time, the oceans were already a busy and dangerous underwater forest, compared to the terrestrial environment that was still free of any movable multi-cellular organisms. The key was “moving”, because Qciz, the fungus like-creature that was thriving in the coastal rocky pools, had already evolved the ability to produce drought and cold resistant spores that could survive outside of water for multiple moon-decades.
The ancestor of Qciz was once a generalist parasite that could “possess” most oceanic creatures. The neutrally buoyant spores of their ancestors, once having entered the host via any openings, which were usually the gill-like structures, would germinate within 4 to 12 hours, extending out its hyphae and trying to find the shortest path to the brain. Most healthy organisms had the capability to dislodge or eliminate those spores before they germinated, and most newly germinated fungi died before reaching the brain. However, the actual target of the spore was those injured or distressed animals that often lacked the ability to defend against it. If the fungus could successfully enter and infiltrate the brain, then it would slowly devour the brain. However, instead of killing the host directly, it would consume the host’s brain and produce a pair of gametophytes that had encoded incomplete fragments of the host’s genome, primarily the genes related to the host’s nervous system.
Once the gametophytes start to develop, instead of eating the host, they would perform matriphagy, meaning that they grow by sucking the nutrients from their parent. For a short period of time, the fungal parent, the gametophytes, and the decaying brain would be simultaneously present in the same cavity.
Containing genetic codes from the hosts, the gametophytes were tasked to growing into a pseudo-brain that retained almost all physiological functions of the host’s original brain. In fact, the gametophytes were tricked into identifying itself as part of the host’s body. Before the host’s brain had been fully consumed, the gametophytes would have essentially replaced the host’s brain, becoming the new brain that controlled the host’s entire body. At this stage, the host had basically become zombified, as it could still move, eat, hunt, and even engage in reproductive activities; except it had lost all cognitive functions, at least from the perspective of the victim. The gametophytes did so because it could continue to grow only by absorbing nutrients that were sent to the “brain” via the circulatory system, and the host needed to stay alive long enough for the gametophytes to sexually mature and start producing spores, thus completing the lifecycle.
Despite its gruesome lifecycle, it was a rare fungus in the ecosystem. First of all, there were many planktonic animals that consumed those spores. Second, most animals that fell victim to the fungus were already injured or sick. Third, the zombified hosts, despite being able to carry out the basic bodily functions, had lost most if not all of its cognitive and defensive abilities. For example, a zombified ambush predator would start wasting energy chasing preys aimlessly, or a zombified hermit crab-like creature might suddenly come out of their shell, exposing itself to the predators. Therefore, even after discounting the host’s initial injuries and sickness, the zombified host did not always survive long enough for the gametophytes to produce spores.
Qciz, guardian of the land
Throughout the course of millions of years, different subspecies of this zombie fungus began to arise after specializing in different hosts and microenvironments. Qciz, likewise, was one of those subspecies that specialized in zombifying the small aquatic animals that resided in the coastal rocky pools. Although the zombified hosts could generally survive longer periods of time, thus increasing the likelihood of the gametophytic Qciz to sexually mature and produce spores, the fact that these rocky pools routinely dried up meant that the spores of Qciz could easily be destroyed simply due to the dry and cold environment on land. Over time, Qciz started to produce sturdier spores that could still germinate after being blown inland for moon-decades, away from the ocean. In other words, it became the first “fungus” that could not only spread to new hosts underwater but also out of the water. Over the next hundreds of millions of years, the waves and winds had carpeted the land with Qciz spores.
This newly evolved amphibious life stage was exceptionally crucial, because by the time that Qciz had escaped the boundary of water, there still hadn’t been a truly terrestrial animal species that roamed the land. At that time, multiple amphibious or semi-aquatic animals had emerged, routinely consuming detritus along the shore or escaping from the aquatic predators, but not a single species could fully adapt to living its life entirely out of the ocean. However, that soon changed when something special happened between Qazho and Qciz.
Qazho was one of those semi-aquatic animals that was living in the intertidal zone. It was a generalist omnivore that consumed virtually any nutritious food items – whether dead, alive or poop – in the shallow water and on the coast. Like the other animals, Qazho would occasionally become zombified. In fact, the animals would contract Qciz a lot more easily on land, because if a spore came into contact with the animal while it was out of the water, it would be glued to the host due to surface tension, similar to how wet sand stuck to objects more easily than dry sand. However, once the animal became zombified and started consuming detritus on the coast, it would basically be an instant death sentence to both the fungus and the host. That was because one of the most common detritus was Qowaris’ faeces, which were highly toxic.
Qowaris was not a single species, but the collective term of a bunch of distantly related zooplanktons in the ocean. Qowaris were unicellular or multicellular animals that were capable of digesting and extracting the energy from primary producers. In other words, even though Qowaris were not primary producers themselves, they single-handedly powered the entire ecosystem, giving life to the higher trophic levels.
In the ocean, the faeces of Qowaris were actually non-toxic and very nutritious; however, once it got washed up to the shore and became dry, one of the waste compounds would interact with the atmosphere and undergo a series of chemical reactions, finally producing a lethal toxin that would directly kill both the animal and the fungus when consumed in large quantities. All the amphibious and semi-aquatic species, if they were detritovores at all, had learnt to instinctively avoid the dried faeces of Qowaris. However, a zombified animal would simply ingest the faeces indiscriminately along with any food items they had found along the shore.
The first animal to finally acquire resistance to the toxin was the Qazho, but even then it was a painfully long process — around a billion years indeed. Since Qazho and the other animals were easily infected by the Qciz spores out of the water, they had learnt not to stay out of the water for long hours. As long as they routinely went back into the water, the spores that had “adhered” to their wet bodies would come loose and dislodge before it could germinate. Those who spent too many hours on the land would become zombified and subsequently poisoned.
For the next billion years, Qciz was essentially the “guardian of the land”, discouraging animals from becoming true terrestrial. Funnily, Qciz not only protected the land from the terrestrialization of animals, but also from itself, since the spores would not germinate without a host.
The real glass ceiling
The peculiar biotic relationships surrounding Qciz were what delayed the terrestrialization process by a billion years.
From the perspective of an individual animal, Qciz was definitely an unwelcomed guest. However, it was by far not the worst way to go. First of all, Qciz was largely a non-threat to the healthy individuals, which were the majority of the populations. Even if a sick animal did become infected and zombified by Qciz, it only meant that the cognition of the animal was gone, but the fitness was not immediately lost; in fact, some zombified animals even had a higher mating rate because they wouldn’t consciously resist courtship attempts, and the gametes or embryos of the zombified animals still remained completely healthy and normal.
As a result, not many animals had developed successful defensive strategies against Qciz; or to be more precise, not many had attempted to develop any defense against it in comparison to other significant biotic and abiotic threats from the ocean. Even the Qazho, which were frequently zombified by Qciz on land, did not develop defensive mechanisms to protect themselves from becoming parasitized. Instead, they diverted their energy towards resisting the toxicity of Qowaris’ poo.
Building up resistance to the toxin was, in a way, similar to building up tolerance to cyanide. If eating too much cyanide would instantly kill you but eating just a little bit wouldn’t, then, theoretically, your body would slowly be able to withstand more and more cyanide if you started eating just a little bit of it every day. Over time — probably hundreds of generations to be precise, your great great great grandchild might just unlock new antitoxic pathways and tolerance levels to cyanide via mutations.
However, this whole theory is hanging on the assumption that you are constantly exposed to cyanide in close proximity. In the case of Qazho, they never needed to consume the toxin. Even though Qowaris’s faeces were the most abundant detritus on the shore, there were more than sufficient food sources without it. A healthy Qazho would simply avoid the intoxicating excrement at all, and a zombified Qazho would easily consume more than the lethal amount. Either way, it was statistically improbable for any semi-aquatic animal to be exposed to just the right amount of the toxin to live and tell the tale; even if that happened, once-off events like that could hardly leave any meaningful genetic footprints to be cascaded down to the next generations.
This was perhaps why it took so long to happen. The first Qazho that could withstand the toxin and pass on the trait.
But at some point, it did happen.
Enemy with benefits
A pregnant female Qazho, somehow, hit all the correct buttons. It went out of the water for food and shelter but had returned to water well within hours. Unfortunately, one particular spore remained lodged in its mouthpart. Unfortunately, it germinated and had infiltrated the brain. Unfortunately, its new “brain” wanted to go back on land for some sundried Qowaris’ faeces. Unfortunately, it ate too much of it.
But it didn’t die.
Perhaps it was the accumulative contribution of its ancestors, or perhaps it had some kind of a genetic mutation — it just didn’t die.
Even though the host’s body could survive the toxin, it was still highly toxic to Qciz. Surely enough, its zombie brain couldn’t withstand the toxin and began dying. However, the pregnant Qazho released all of her eggs before it became brain dead.
Some of the Qazho’s eggs hatched, and three of them retained their late-mother’s resistance to the Qowaris fecal toxin. The timeline had been knocked up a gear from that moment onwards. Within just 200 moon-years, 0.5% of the local Qazho population were completely resistant to the toxin. However, just because those Qazho were resistant to the toxin, they remained behaviorally indifferent to consuming dried Qowaris faeces or moving on land.
Ever since the first Qazho became resistant, there had been a seemingly trivial yet substantial change between Qazho and Qciz. Previously, a Qazho that had spent too much time on land would become zombified, and then it would consume too much Qowaris faeces and die. Now, if a resistant Qazho spent too much time on land, it would become zombified and consume the Qowaris faeces without poisoning its own body; but it would still die by brain death. When a Qciz spore germinated and consumed its host’s brain, it’d copy fragments from the host’s genome and pass them on to the gametophytes, but so far, no Qciz gametophytes had been able to copy the genes associated with toxin resistance. What had changed, though, was that it’d only be a matter of time until Qciz “inherited” the trait from their Qazho hosts.
In fact, it only took two moon-years for the long-awaited event to arrive.
It was another female Qazho that had inherited resistance to the toxin. This time, the Qciz fungus finally copied those exact genes and passed on to the gametophytes which later replaced the Qazho’s brain. In face of an influx of the toxin, the Qciz pseudo-brain would become slightly irritated, but there would no longer be any lethal damage to the brain or the host.
On the contrary, because of the minor irritation from the toxin, the Qciz tripled in metabolic rate, becoming hyperactive and drawing in extra energy from the host. In order to compensate the increased energy demand, the pseudo-brain now signaled its host to consume more organic material, in turn leading to a heightened rate of ingestion of the toxin. This created two immediate changes to the zombified Qazho female.
First of all, at least tens of Qazho individuals that were resistant to the toxin had been zombified by the fungus, but this female was the first one to survive a prolonged period of time instead of slowly becoming paralyzed due to brain death. Second, Qazho was originally a rockpool species that ONLY consumed food items present in the coastal zone; they used to actively avoid consuming the dried-up Qowaris faeces because of the deadly toxin. The only instances where a Qazho would accidentally ingest a large amount of the toxic compound was when it became zombified. This particular female, stripped of all Qazho-specific cognition and instinct, instead favored the wildly abundant Qowaris faeces on land. The sudden removal of dietary restriction paved a new evolutionary path for her future descendants.
Additionally, the mutated Qciz strain also encountered new ecological pressure. Originally, the Qciz gametophytes would take around 15 moon-days to complete its life cycle. Although the mutated Qciz had a higher metabolic rate, most of the energy was actually used to neutralize the toxin and repair the micro-damages, leaving less energy for the reproductive development. It would now take more time for Qciz to reach sexual maturity and produce fertilized spores. On the other hand, although Qciz and Qazho shared the same toxin-resistance gene, their actual resistance differed significantly. The gene coded for a compound that acted as an enzyme to breakdown the toxin into harmless compounds. Such was the case for Qazho. However, Qciz had an additional pathway that would first metabolize the toxin into a secondary metabolite, which was even more toxic. Qciz naturally produced this metabolite in trace amount, and it could also break it down into harmless compounds and water. Qciz was naturally tolerant to this metabolite but was not completely resistant to its toxicity.
To give a similar example, the human body naturally produces alcohol (ethanol). To digest, the liver first breakdowns the ethanol into acetaldehyde (the compound that causes hangover and alcohol flush), then further catabolizing it into harmless compounds. Both ethanol and acetaldehyde are toxic in high concentration, although the human body has become very tolerant through generations of alcoholic lifestyle. For Qciz, the altered metabolism and constant intoxication produced very substantial changes that were not visible until many generations after.
To be inseparable
The first Qazho to truly survive independently on land was technically not a Qazho, but a creature that had the body of a Qazho and the brain of a Qciz. Emphasis on the pair, because later the first migrants to inland area were all zombified Qazho.
By the time that the first resistant Qciz produced fertile spores after approximately 30 moon-days, which was double their normal span, the female host had laid two clutches of eggs. Despite not having cognition, she foraged and spent more time on land than in water, thus avoiding most predators and competitors. The first clutch was laid just outside of the intertidal range and therefore soon perished due to desiccation. The second clutch was luckily submerged in the rockpool and the eggs hatched successfully.
Normally, the Qciz spores that were produced in the host’s brain cavity would be transported back to the gills and naturally carried away by water. However, since the host stayed and died on land, the spores could not be dispersed into the water. The spores remained inside of her head even after she was completely wind dried. They were only released when another Qazho came across her carcass and ate her open. Some of those spores were stuck onto the new individual while the rest dispersed with the wind. This new method of spore dispersal — through cannibalism — turned out to be the norm in the coming generations.
By now the only creatures that had developed the tendency to spare extensive life stages on land were the toxin-resistant Qazho that had been infected by the toxin-resistant Qciz. At this point, the relationship between these two organisms could be considered, in a way, symbiotic. In terms of general survivorship, the pairing was facultative for either species. But specifically for the transition to a terrestrial lifestyle, it was obligate for this duo to pair up.
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[Plant ecology] Sporophyte, gametophyte & the alternation of generations
Plants, animals and fungi on Earth generally have two phases. Humans, for example, we are the diploid phase (the “DNA” in each of our cells are all doubled up and in pairs), and our sperms and eggs are haploid (the “DNA” in each sperm and egg cell are single and alone).
If you look at human reproduction on the cellular level, you can see that humans go from diploid to haploid and then back to the diploid phase.
The haploid phase in humans is very short and brief, as the sperms and eggs cannot grow to become their own animals. Their only option is to fuse to form a diploid embryo (also known as a zygote).
However, plants behave very differently in what we call the alternation of generations in biology. A textbook example is ferns.
Ferns, like humans, are the diploid phase. But there are no boy ferns and girl ferns, and ferns can’t produce haploid sperms and eggs. Instead, mature ferns release haploid spores which germinate to become a very different looking plantlet. This small plant is called the prothallus and is haploid. It has both male and female reproductive organs, but there are mechanisms to prevent self-fertilization (i.e., fusing your own sperm with your egg). After the prothallus fertilize with one another, the fertilized embryo (i.e., the sporophyte fern) will grow out of the prothallus. Eventually the prothallus will die and only the fern will continue growing (see the pictures above).
In simple words, the diploid fern gives birth to haploid prothallus, then the haploid prothallus fertilizes (i.e., pairing of sperm and egg) and gives birth to the diploid fern.
In plant ecology, the diploid organisms are called sporophytes, which only produce spores. The haploid organisms are called gametophytes, which only produce gametes (aka., sperms and eggs). Plants like ferns go through both the sporophyte and the gametophyte phase to complete a single life cycle. Hence, it’s called the alternation of generations.