Along the eastern Atlantic coasts of France, at some point in the last 100,000 years, two ribbon worms of different species engaged in worm intercourse (do not fear, I will not discuss the mechanics here). The two species were Lineus sanguineus and L. lacteus. Interspecies sex is uncommon in itself, but what’s especially surprising in this case is that their union gave rise to a new species: L. pseudolacteus.
Several barriers to interspecies sex have long been documented. Often there are no offspring from these unions. Even when the species are closely related enough to give rise to a hybrid, most hybrids are infertile, like mules and hinnies, which are born when horses and donkeys are crossed. On the other hand, some hybrids can be fertile. For example, oranges, which are hybrids of pomelo and mandarin are often fertile. However, our main protagonist, the hybrid of L. sanguineus and L. lacteus, is infertile, yet it can reproduce… by a special kind of regeneration.
When thinking of regeneration, we often think of earthworms. You may have played with them as a child, and perhaps you were even told earthworms do not die from being halved. This is true, but only the head part can regenerate a tail, while the tail half dies. In this way, earthworms have a limited capacity for regeneration.
Conversely, regeneration in some ribbon worms can happen with both halves! This type of reproduction is called fission, fissiparity or (clonal) fragmentation, depending on the organism involved. In the case of unicellular organisms such as bacteria, this process is called fission, which is very common. In mushrooms and plants it’s called fragmentation and it is also common; clonal fragmentation is the basis for multiplication by cutting. Although much rarer in animals, fragmentation–which is called fissiparity when referring to animals–can happen in corals, sponges, starfish, annelid worms, ribbon worms and a few others. Our good friend, L. pseudolacteus is one of those animal species that can regenerate by fissiparity.
L. pseudolacteus inherited two sets of chromosomes from L. sanguineus and one from L. lacteus. Normally, one set is inherited from the father and another set from the mother. As a result, our hybrid has three sets of chromosomes rather than two. Having different numbers of chromosomes often leads to infertility in hybrids, and makes L. pseudolacteus incapable of reproducing sexually. However, it can reproduce asexually by fissiparity. It inherited this ability from its mother species, L. sanguineus, which can reproduce either sexually, or by fissiparity, while its father species, L. lacteus, can only reproduce sexually. This discovery was published in July 2016; Dr. Eduardo Zattara, a former post-doc at IU’s Moczek Laboratory in the Department of Biology, and a co-author of the study, has recently returned to work in his homeland of Argentina.
This is the end of my story on our friends the regenerative ribbon worms, but this study raises exciting possibilities for research on the effects of asexuality on genomes. Sex is a very useful way of ‘shuffling the cards’ genetically, thereby getting ‘better hands’—better combinations of genes in individuals, which then leave more descendants. A species that has recently stopped this genetic shuffling can teach us much about asexual genome evolution.
Reference: Ament-Velásquez, S. L., E. Figuet, M. Ballenghien, E. E. Zattara, J. L. Norenburg, F. A. Fernández-Álvarez, J. Bierne, N. Bierne, and N. Galtier. 2016. “Population Genomics of Sexual and Asexual Lineages in Fissiparous Ribbon Worms (Lineus, Nemertea): Hybridization, Polyploidy and the Meselson Effect.” Molecular Ecology 25 (14): 3356–69. doi:10.1111/mec.13717.