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VOLVOCINE ALGAE have a recent evolution of multicellularity, only 30-70 million years ago. This may produce a better record of the early history of this process than we have for other multicellular organisms. Metazoans and multicellular plants evolved over 550 million years ago (multicellular plants multiple times, and some suggest a very ancient history of multicellular algae over 800 million years ago). The fossil record for fungi is not very good, but unambiguous multicellular fungi were present by 500 million years ago. Bacteria meanwhile beat everyone out by evolving multicellularity several times perhaps 2-3 billion years ago. Since most multicellular organisms have a distant origin, extinction has eroded the base of their evolutionary trees so that the details of the transition are hard to extract. The volvocine algae have a much more recent history of multicellularity, and we have been able to determine much about their evolutionary history from phylogenetic studies of these algae and their relatives.
IN THE PAST week or so I’ve been writing about the attine ants, which have a complicated mutualistic network combining cultivated fungi and actinomycete bacteria, and are parasitized by Escovopsis fungi and perhaps black yeasts as well! Today I’m writing about the attine ants again, but along a very different angle. In this case this paper examines the influence of evolution upon reproductive behavior. I actually ran across the paper by accident, having previously planned to write about a paper on evolution and reproductive behavior in humans. This paper nicely transitions between these two themes.
Among organisms in general it is a bad idea evolutionarily to abandon breeding in favor of helping another individual raise its offspring. There are examples of social cheaters among groups as different as myxobacteria, slime molds, vertebrates, and insects. The myxobacteria and slime molds are bacteria and eukaryotes respectively that have converged upon a similar lifecycle. These are organisms capable of lone existence, but which mass together during unfavorable conditions to produce a stalk that launches spores. Ideally every strain in the group will be represented equally in the spores produced, but some cheater strains are able to produce more than their share of spores. Among vertebrates, there are examples in many groups of species that parasitize the nests of others, foisting off their young upon an unsuspecting individual. Cowbirds and cuckoos are well known among the birds, and cuckoos gave their name to the cuckoo catfish, which parasitizes cichlids. A similar parasitism is seen among insects where some wasp species will infiltrate another species nest and lay their eggs. The ants take this type of parasitism even further, with some species going to war against others to capture their larvae, which are raised in their captor’s colony and tend their brood. But the type of reproductive cheating occurring in attine ants is different from all of these!
I HAVE ALREADY mentioned one type of selfish genetic element. These are mobile elements that can move about and reproduce within the genome, and include the transposon and retrotransposons. A second similar type of selfish genetic element are the homing endonucleases. These come in two forms, as introns which are spliced out of RNA and then translated into protein instead of being discarded, or as inteins that splice out of the protein once it has been synthesized. In both cases the homing endonuclease then during meiosis attacks the allele that does not contain the homing endonuclease intron or intein and triggers DNA repair that duplicates the homing endonuclease’s sequence. Since mobile elements and homing endonucleases either attack at a wide variety of sites or duplicate onto both of a pair of chromosomes, they are passed on according to Mendelian inheritance patterns. But there are other selfish genetic elements that are passed on preferentially, and a new paper in Genetica focuses on the effects of these selfish elements upon fertility in carrier males.
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