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A different kind of Transitional

Alec MacAndrew

Transitionals are usually regarded as extinct fossil species that occupy a space between major groups and which represent the process of transition between these groups. So, for example, the transition between marine and land vertebrates is represented wonderfully by a number of clear transitionals such as Acanthostega gunnari, Ichthyostega , Tulerpeton, Pederpes finneyae and Tiktaalik (see also Jennifer Clack’s superb book on the marine:land transition, Gaining Ground). Transitionals are, of course, not just represented in fossils – all species are in transition between the ancestral form from which they came and the evolved form to which they are headed.

A recent paper in Science  (Nakabichi et al,
Science 314, 267) seems to me to represent a different sort of transition in action. Lynn Margulis (whose views on other matters such as the Gaia hypothesis and her claim that there is no scientifically demonstrated link between HIV and AIDS shows just how much tolerance good science shows to those with a mixture of good and bizarre ideas) developed the now widely accepted idea that organelles (such as mitochondria which have their own DNA) in the cells of eukaryotes (animals, plants, fungi and protists) originated as bacterial endosymbionts. Endosymbionts are separate organisms that live in symbiosis with a host cell – ie they provide some benefit that the host cell needs and in turn are supported and protected by the host cell. There is strong evidence that mitochondria were originally free-living bacteria, which invaded host cells, perhaps originally as parasites. Subsequently a mutually beneficial relationship developed (there are a vast number of mutually beneficial relationships between bacteria and host organisms, with bacteria providing benefits by their action from mammalian guts to the roots of plants). Endosymbionts make their living on the same principle except that they live within the cells of their host organism. Long lasting endosymbiotic relationships become very close – they get to the point where neither host nor bacterial invader can live without the other. There is good evidence that the bacterial invader over time abandons much of the basic physiology and genetic makeup that enabled it to live independently, because the host cell provides many of those basic functions. Indeed, there is evidence that lateral processes transfer some of the genetic material of the endosymbiont from the genome of the organelle to the nuclear genome of the cell. For example go here.

Endosymbionts are well known today. For example, endosymbionts are known to exist in many varieties of insect cells. In most cases the endosymbionts are restricted to specialised cells called bacteriocytes. They reproduce through generations of the host cells like organelles. The endosymbionts have massively reduced genome sizes and a big bias of nucleotide composition (the four nucleotides, A, T, G and C are approximately equally represented in the genomes of free living organisms but in endosymbionts and organelles the GC content is significantly reduced). Examples of endosymbiotic bacteria in insects include Buchnera, Blochmannia, Wigglesworthia and Baumannia. Nakabachi et al have just published a
fascinating short paper in Science in which they report the sequencing of an endosymbiont, called Carsonella ruddii, that is found in all species of a type of insect called psyllids that feed on plant sap (Pachypsylla venusta). The characteristics of this bacterial symbiont lie way beyond that of other known insect endosymbionts.

How? Well first of all the genome of Carsonella is tiny – it consists of 160 kilobases (which is a third of the smallest previously known bacterial genome), and it contains only 182 genes most of which have some physical overlap with one another. It has a very low GC - guanine/cytosine - content at only 16.5%, way below that of other known organisms. Carsonella has lost all of its genes for many categories that free-living bacteria need such as the creation of a cell envelope and the genesis of nucleotides and lipids. Its genome lacks many genes that are necessary for biological processes of free-living bacteria. It seems that the host cell compensates for this lack of apparently critical function. On the other hand Carsonella is rich in genes to synthesise essential amino acids in which the food (plant sap) of the host insect is poor – this is evidence of the positive function of Carsonella to its insect host. Carsonella is so reduced and so utterly dependent on its host nuclear genome that it can be regarded as a transition between an obligate endosymbiont and a eukaryotic organelle. It is a genuine transitional on its way from bacterium to organelle. Never let creationists tell you that there are no transitionals.