Project 21

Evolution and phylogenomics

 

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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, Center of Tardigrade Research, Switzerland

 

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     

 

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