First published: Technical Journal 14(1):9–10 April 2000 Browse this issue Subscribe to TJ

Jumping wallaby genes and post-Flood speciation

by Pierre Jerlström

Jumping genes or transposable elements (TEs) are present in virtually all life forms, from bacteria to humans. They are short DNA sequences that can move from site to site in the chromosomes of their hosts. They have been divided into two groups, DNA transposons and retroelements.¹

DNA transposons can move by a cut-and-paste mechanism or by making copies of themselves. Retroelement jumping involves copying its RNA transcript back into DNA (see figure) using the enzyme reverse transcriptase. In contrast to transposons, however, most retroelements ‘jump’ only occasionally, and there are many which appear to have lost this ability.¹ One exception is an active fruit fly retroelement.²

Retroelement replication cycle Many RNA copies are transcribed from the integrated retroelement. Some are reverse transcribed into DNA copies, which are reintegrated into the host chromosome. The cycle can then repeat. Boxed regions are retroelements or RNA copies (after Kidwell and Lisch).13

Retroelements have been found in the chromosomes of eukaryotes (which unlike bacteria, have cells with a defined nucleus) such as yeast (the simplest eukaryote), fruit flies, and in vertebrates such as mice and man. The DNA sequence elucidated from the human genome project shows that 35– 40% of human DNA is made up of retroelements.³

Based on evolutionary presupposition, jumping genes were initially believed to be simply ‘selfish’ DNA with the sole function of self-perpetuation, and with no apparent use to the host (junk!).^4,5 But lately, molecular biologists have been unveiling crucial functions for these elements. For example, retroelements have been found to play an important role in regulating gene expression (switching genes on and off) and in the repair of chromosomes.¹

Research on wallabies

In addition, an Australian research group has now proposed that retroelements are involved in mammalian speciation.⁶ The researchers were studying an infertile hybrid wallaby they had found in a wildlife park—a result of crossing two different species: the swamp wallaby and the tammar wallaby. They looked at the genome of this unusual hybrid marsupial, because:

1. there are genetic models that predict that rearrangements in the genomes of hybrids can aid reproductive isolation, by preventing hybrid and parent species crossing, leading to the formation of new species,⁷ and

2. TEs can cause DNA rearrangements in hybrid fruit flies.²

They found that a retroelement had jumped profusely in the genome of the hybrid wallaby, and had integrated around the chromosome centromeres—the region on the chromosome to which the spindle fibres attach during cell division. One of the researchers commented:

‘We thought it took millions of years of long-term selection for a jumping gene to be activated. We’ve now shown that it can happen maybe in five minutes after fertilisation.’⁸

The finding indicated also that the jumping gene in the hybrid had probably been reconstructed (and reactivated) by the genetic combination of two incomplete, inactive segments of a retroelement present in one or both of the parents. This has previously been reported in mice.⁹

Since DNA methylation (the linking of a methyl group to the nucleotide bases adenine or cytosine)¹⁰ is important for switching off genes, it is believed that this process is also used by host cells to stop the movement of TEs.¹¹ Deficient methylation would then enable jumping. The researchers found that the retroelements in the DNA of the hybrid were indeed unmethylated. This was also the case in two additional hybrids, each from an independent mating between two different wallaby species.

The researchers suggested that the ability of retroelements to produce genomic rearrangements in hybrid wallabies, due to deficient methylation (eventually resulting in the formation of new species) may be a widespread phenomenon in hybrid mammals. However, this idea has now been challenged by another research group, who did not find any methylation changes in the genomes of a number of mammalian hybrids they studied.¹²

Evidence supports biblical model

The very rapid genetic changes caused by TEs could help explain the formation of the variants from the original kinds on Noah’s Ark in the relatively short biblical time frame.¹ Rapid speciation apparently occurred, since early historical records already show a large variety of types similar to those present today. The various species representing the variants in e.g. the kangaroo/wallaby created kind would all then stem from the original parent kind present on the Ark.

Based on the rapid jumping of retroelements in hybrid wallabies⁶ and fruit flies,² on their ability to cause DNA rearrangements, and on their role in gene regulation,¹ it is plausible that these TEs were active in the past, and were an original mechanism for expressing God’s programmed variation within kinds. Today, the presence of incomplete, inactive segments of retroelements in wallaby chromosomes, which probably recombined in the hybrid to form active elements, as in the case of hybrid mice,⁹ may be vestiges from this past mechanism.

Are retroelements involved in wallaby speciation today? The wallaby crosses described here only produce sterile males, a result which is common in interspecific hybridization in mammals.¹² Therefore, most male hybrids cannot produce offspring with female hybrids from a compatible crossing. In the unlikely event of a fertile male successfully mating, there is also no certainty that this will produce viable offspring. These obvious problems, and the fact that TEs are mostly inactive today and are only ‘jump started’ by the unlikely events of crossing between species, indicates that the role of retroelements in speciation has been either greatly reduced or has stopped altogether.

Further reading

• Evidence for the design of life: part 1—Genetic redundancy
• Evidence for the design of life: part 2—Baranomes
• Evidence for the design of life: part 3—an introduction to variation-inducing genetic elements
• Evidence for the design of life: part 4—variation-inducing genetic elements and their function

References

1. Walkup, L., ‘Junk’ DNA: evolutionary discards or God’s tools? TJ 14(2):18–30, 2000. Return to text.
2. Petrov, D.A., Schutzman, J.L, Hartl, D.L. and Lozovskaya, E.R., Diverse transposable elements are mobilized in hybrid dysgenesis in Drosophila virilis, Pro. Natl. Acad. Sci. USA 92:8050–8054, 1995. Return to text.
3. Jurka, J., Repeats in genomic DNA: mining and meaning, Curr. Opin. Struct. Biol. 8:333–337, 1998. Return to text.
4. Doolittle, W.F. and Sapienza, C., Selfish genes, the phenotype paradigm and genome evolution, Nature 284:601–603, 1980. Return to text.
5. Orgel, L.E. and Crick, F.H.C., Selfish DNA: the ultimate parasite, Nature 284:604–607, 1980. Return to text.
6. O’Neill, R.J.W, O’Neill, M.J. and Marshall Graves, J.A., Undermethylation associated with retroelement activation and chromosome remodelling in an interspecific mammalian hybrid, Nature 393:68–72, 1998. Return to text.
7. Templeton, A.R., Mechanisms of speciation—a population genetic approach, Annu. Rev. Ecol. Syst. 12:23–48, 1981. Return to text.
8. Research in Action, La Trobe University, pp. 7–8, 1998. Return to text.
9. Jaenisch, R., Schnieke, A. and Harbers, K., Treatment of mice with 5-azacytidine efficiently activates silent retroviral genomes in different tissues, Pro. Natl. Acad. Sci. USA 82:1451–1455, 1985. Return to text.
10. Jerlström, P.G., Genomic imprinting, TJ 13(2):6–8, 1999. Return to text.
11. Yoder, J.A., Walsh, C.P. and Bestor, T.H., Cytosine methylation and the ecology of intragenic parasites, Trends Genet. 13(8):335–340, 1997. Return to text.
12. Roemer, I., Grützner, F., Winking, H., Haaf, T., Orth, A., Skidmores, L., Antczack, D. and Fundele, R., Global methylation in eutherian hybrids, Nature 401:131–132, 1999. Return to text.
13. Kidwell, M.G. and Lisch, D.R., Transposons unbound, Nature 393:22–23, 1998. Return to text.

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