Evolution and phylogenomics |
Tardigrades. Do they have evolved with some other
metazoan phyla from hybridized ancestors? or : May natural hybridization generate animal lineages
with phyla characteristics? Markus Dicht, Abstract The
evolutionary importance of speciation by natural hybridization in animals is,
till now, unknown. The consensus is that all phyla arised from single-species
ancestors. Hybrid origins of new phyla are uncommon. But there are examples
of natural hybridization leading to speciation in animals. Although there are
reported cases of homoploid speciation events among fishes, mammals, insects
and other invertebrates there is lack of empirically based research. We
cannot exclude that reticulated evolution by hybridization may be a viable
mechanism for speciation or the generation of new invertebrate phyla in the
early evolution of Metazoa in the
(Pre)Cambrian. Conventional
cladistic analysis produce only divergently branching phylogenetic patterns.
If an analysis would include hypothetical hybrids a traditional cladistic
method can never give the correct phylogeny, no matter where the hybrid is
placed. Traditional methods or algorithms can give confusing and conflicting
results as we have often seen in the past. Hybridizations between internal
branches cannot be analyzed by the traditional algorithms with satisfaction. Too little
attention was previously payed to the fact that mitochondrial genomes are
mostly of maternal origin and that the nuclear genes are of mixed parental
origin. Furthermore it is striking that some representatives of ‘minor phyla’ have adult features and others (e.g.
Tardigrada, Loricifera,Kinorhyncha or Rotifera) more larval features
(pedomorphic phenotypes). A possible importance of neoteny or progenesis in
the evolution of invertebrate animals was mostly excluded in
phylogenetic studies till now. Perhaps we
should postulate in future a ‘pattern pluralism’ for the Tree of Life because
whole genome comparisons will reveal a possible chimeric origin for some
animal phyla. The
primary question of the CTR-studies was : What was the phylogenetic origin
(common ancestor) of the phylum Tardigrada ? In the course of numerous
molecular and other phylogenetic analyses
the following problem appeared more and more : The possible importance
of hybridogenesis in invertebrate evolution. 12
mitochondrial and 19 nuclear genes were used for the CTR-studies. Not only
BLAST and Fast Minimum Evolution or Neighbour Joining methods were used to estimate evolutionary relationships
between sequences, but also other methods combined with different other
characters. For the
following phyla hybridized ancestors could be hypothesized : Tardigrada,
Nematoda, Onychophora, Priapulida, Kinorhyncha, Rotifera, Loricifera,
Micrognathozoa, Gnathostomulida, Cycliophora and Entoprocta. It could be
hypothesized that interphyletic crosses between certain marine animals like
Polychaeta, Crustacea (e.g. Pentastomida- , Remipedia-, Cirripedia- or
Malacostraca-like), Mollusca and Echinodermata might have lead to putative
hybridized ancestors of the mentionned phyla. Following
the studies of the CTR it can be hypothesized that the universal Tree of Life
may not only show dichotomously branching, but sometimes for certain phyla,
reticulation, what is possibly a more realistic but complex fact of nature.
So long as the possible importance of hybridization in evolutionary processes
is underestimated or refused ( because of Haldane’s rule or the existence of
different barriers for the transfer of genes between species) a lot of
questions regarding the branching pattern of the Animal Tree of Life may
remain unanswered. Successful hybrid speciation may be a very rare process in
the evolution of animals. But only very little successful chance
interphyletic hybridogenetic events
were needed within a long period of time of several million years, perhaps in
the (Pre)Cambrian, for the formation and the evolution of some new animal
phyla. In marine
intertidal zones eggs and sperm of thousands of species of different phyla
are often cast into the open water where fertilization between unlike animals
seems more probable. There were and are possibly only few pre- or postzygotic
barriers to interspecific breeding or no-choice crosses combined with mass
spawning events of different marine invertebrate phyla. Hybridizations
are possibly much more rapid or fast than gradual transformations of whole
lineages. Understanding interspecific hybridization presupposes much more
knowledge of genome restructuring mechanisms and evolution by ‘natural
genetic engineering’ in response to hybridization. The three
consequences of such a ‘hybridogenesis hypothesis’ may be significant : 1. Neither
only GRADUALISM nor only PUNCTUALISM 2. Neither
only TREE nor only
NETWORK 3. Neither
only ECDYSOZOA nor only
ARTICULATA If you
need more detailed informations about the original text of this abstract, the
methods used, the results and the references, please, contact the CTR
E-mail : info@tardires.ch |