Tree clones: immortality and sex

The oldest organisms on Earth are clonal organisms. Plants seem to have the capacity for immortality because they have perpetually juvenile tissues (meristems) capable of continued growth, and further, certain plant cells can dedifferentiate, return to a juvenile state, for wound repair and growth. This capacity is widely used in the vegetative reproduction of economically important plants. But can a clone live forever? Probably not, but they can be very old. The Pando clone of quaking aspen in Utah is estimated to be at least 80,000 years old, and some estimates place its age as 10 times older, which would make this clone as old as our whole species! Not only is Pando impressively old, it’s big covering over 40 hectares and at over 6000 tons is the largest organism alive. Now of course one problem with being an immobile species is that the longer you live the more likely you’ll encounter some environmental disaster, a fire, a flood, a volcanic eruption, a storm, a chain saw. But clones have a sex problem. Since they are all one genetic individual although looking like a whole forest, they cannot produce seed except by exchanging pollen with another individual, and when one individual occupies the whole area that becomes less likely. Even worse recent research suggests that as the clone gets older, it loses sexual vitality because of an accumulation of mutations. These show up in pollen because pollen only has one copy of each gene so if a harmful mutation occurs it may affect the viability of the pollen, a harm that does not affect the tree because in its tissues chromosomes and genes occur in pairs. By the relatively young age of 20,000 fertility can be diminished by more than 3/4s. So the clone may live a long time, but as it ages, its ability to sire offspring and start a new individual drops, but who knows Pando may already have lots of offspring.