Índice

     Earlier this year there were reports on a study that showed how two different species of butterfly could hybridize to produce a third. This is truly amazing, but could it occur in something other than an insect, say, maybe a vertebrate? If so, how exactly does it work?

    The answer is that it can and, more importantly, that a similar phenomenon (not identical, because the offspring don't breed with each other, although they do reproduce to make more hybrids like themselves...read on!), known as hybridogenesis, does indeed occur in vertebrates. There are two fascinating species of Mexican livebearing topminnows, Poeciliopsis monacha and P. lucida, that are known to hybridize and produce offspring which then reproduce themselves parthenogenetically.


    In this case, the hybrid does not have its own scientific name, it is simply notated as P. monacha x lucida, although there is another example from this genus, possibly the topic of a future post, in which the hybrid has indeed been given its own name.

    So, down to business, what is the story about how these two separate fish species manage to hybridize and yield offspring which can go on to reproduce themselves? Fish in the famile Poecilidae are known as the "livebearers" because they do not deposit eggs, but house them in their bodies until they hatch. While they are commonly referred to as ovoviviparous, it is important to note that different species show different methods of nourishing the young within the body. Some are purely lecithotrophic, meaning that the offspring gain all their nourishment from materials within the egg, but some show varying degrees of matrotrophy, in which they obtain nutrients from the mother's body.

    This is of particular interest, because the species in question show markedly different styles of nourishing their young: P. monacha is considered to be purely lecithotrophic, while P. lucida shows a moderate level of matrotrophy. Studies have shown that among human couples, disagreements over how to raise children are the biggest source of marital conflict. As any bickering human couple could probably attest, it's hard to raise a kid if you can't even agree on how to feed it, adding another fascinating layer to the story of how these fish not only reproduce successfully, but create offspring that are themselves fertile.


    The first step in the process is for the initial event, mating between a male P. lucida and a female P. monarcha. The cross is sex-specific, it doesn't happen successfuly the other way around. Male P. lucida will actively court the females of the other species, and can apparently succeed in attracting them for a copulation.

    Now we have fertilization, and hybrid offspring. How are these offspring going to overcome the fact that they possess chromosomes from two different species? P. monacha has 48 chromosomes, while populations of P. lucida have been shown to have either 48 or 72, but in both cases they can hybridize successfully.

    In many cases, hybrids are sterile for a simple reason: chromosomes. For example, mules are almost always sterile (almost, because about one in a million female mules are fertile, although there have been no cases of fertility in males). This is because donkeys have 64 chromosomes and horses have 62, yielding offspring with 63. Dividing chromosomes evenly is a crucial part of meiosis: when you have an odd number, it just doesn't work correctly. This is the best known explanation for many cases of hybrid sterility.

    How to overcome the chromosome problem? The key here is a phenomenon known as meiotic drive. This happens when alleles become tightly linked, violating Mendel's law of independent assortment and leading to over-representation of genes from a single parent in gametes produced by an offspring.

    When chromosomes line up along the metaphase plate during meiosis, we expect to see a random mix of maternal/paternal alleles on each side. Due to meiotic drive, however, P. monacha x lucida cells show distinct segregation of maternal and paternal cells: all P. monacha cells on one side, all P. lucida cells on the other side. The cell then undergoes an uneven division, producing a daughter gamete (which gets all the "good stuff": organelles, energy, cytoplasm, etc) and a polar body, which is discarded. This is the crucial event: the polar body contained the P. lucida genome, and the haploid cell (destined to become the egg) contains the P. monacha genome.

    So what has effectively happened: the two species hybridized, which should have mixed their genomes, leading to offspring with sexual identity issues which will never be able to reproduce. BUT, we have meiotic drive, which allows the offspring to sort the genomes out, toss dad's genes, and produce eggs identical to the P. monacha eggs they hatched from in the first place. So even though the P. monacha x lucida individuals are hybrids, reproductively they are still P. monacha, if you go by their gametes alone.

    The story isn't over yet, though, there's another twist. The P. monacha x P. lucida individuals are always female, and can only produce females. It doesn't take an experienced biologist to realize that there are going to be no hybrid x hybrid crosses here. So how does the all female line perpetuate itself?


    Interestingly the hybrid animals all choose to mate with male P. lucida, and the males appear to prefer the hybrids over their own species! So the all female line can continue as long as it keeps mating with those dupey P. lucida, the chucking out the male genes and putting forth only their female P. monacha genome for reproduction. This is not a clonal line of females, however, they are hemiclones. They all carry the same maternal genome, but can get a paternal genome from any P. lucida male they mate with, so all the individuals within the hybrid population will not be genetically identical (although their gametes will be).

    This process, again, is known as hybridogenesis. Poecilids actually show a truly amazing variety of reproductive modes, including a way to produce lines of true female clones instead of the hemiclones produced here. Stay tuned for the details of that process in an upcoming post!

    By the way, if you're interested in learning more about hybrid species, Messy Beast has tons of information. It is focused on felids, but also covers a variety of mammals and birds, I highly recommend it!