A genome carries more than instructions for making an animal’s proteins. It also contains the accumulated descendants of DNA sequences that copied themselves. Some of those descendants can produce further copies; others survive as broken, inactive remnants. Counting them all together can therefore hide a major difference between a busy process and the debris it left behind.
For giraffes, that distinction had been difficult to make. Their genomes contain two major families of potentially self-copying sequences, called LINE1 and RTE. Malte Petersen and colleagues asked which was still contributing new insertions. Their 2021 study in Mobile DNA found a strong contrast: LINE1 remained the dominant source of recent mobile-DNA variation, whereas RTE showed signs of decline.
A genome can contain its own copying machinery
The sequences at the centre of the study are retrotransposons. A working copy can be transcribed into RNA and then copied back into DNA at another position. When that happens in the lineage of cells that produces eggs or sperm, the new insertion can become part of a descendant’s inheritance.
The original copy need not disappear. Repeated rounds can fill a genome with related sequences. Mutations subsequently damage many copies, and the copying process itself often produces shortened versions. A large family of old fragments may consequently occupy plenty of DNA while having very little capacity to expand further.
Petersen’s team first annotated repeated sequences in a Kordofan giraffe reference genome. Repeats covered about 44.6% of that assembly. That established the scale of the material under investigation, but a single assembled genome could not reveal the whole recent history. For that, the researchers needed to compare individuals.
The informative difference is an insertion’s address
The team analysed existing sequencing data from 48 giraffes across four species. They searched for locations where an insertion was present in some genomes and absent in others, using procedures that could detect both additions relative to the reference and insertions missing from an individual.
This comparison matters because a reference genome is one animal’s sequence, not the complete inventory of a species. An insertion absent from that reference can still occur widely elsewhere. Comparing both directions makes the inventory less dependent on which giraffe supplied the reference.
The broad filtered screen contained 139,525 insertion calls: 121,237 LINE1 and 18,288 RTE. LINE1 contributed about 87% of that total. These are genomic observations across the dataset, not counts of copying events watched as they happened in living animals.
A second comparison made the interpretation more persuasive. More than 6,400 recent LINE1 insertions were approximately full length, compared with only 59 for RTE. Full length alone does not guarantee that a copy works, but extensive shortening can remove machinery required to make further copies. The contrast therefore concerns the ability to sustain the process, as well as the amount of variation it has produced.
Old RTE copies may be running out of successors
The researchers looked more closely for RTE sequences with intact protein-coding machinery. A few potentially functional copies remained. Most of those examined appeared to have entered the giraffe lineage after its separation from okapi but before the diversification of living giraffe species.
That places the remaining candidates in an older generation of insertions. Meanwhile, most recent RTE insertions were shortened, making them unlikely to become effective new sources themselves. Small dependent sequences that rely on RTE machinery also showed extremely little recent activity.
Together, these findings support a process gradually losing its capacity to renew itself. The authors proposed that RTE could eventually become inactive in the giraffe lineage. They did not demonstrate that every copy had already stopped functioning. Their inference comes from the ages, completeness and distribution of DNA sequences, rather than a direct laboratory assay of every candidate.
Assembly quality also mattered. Two assemblies made from the same Masai giraffe produced different counts of intact-looking RTE sequences because gaps interrupted some copies. That internal comparison explains why the team combined several kinds of evidence instead of treating one genome’s count as definitive.
Insertions also carry a record of ancestry
An insertion has an address as well as a sequence. If several giraffes share one at the same genomic location, inheritance from a common ancestor can explain that shared pattern. The team used a selected set of insertion locations to compare population relationships and genetic variation.
The broad groupings generally agreed with analyses based on single-letter DNA differences. This was a different kind of genetic marker extracted largely from previously studied animals, rather than a new independent wildlife sample. Some relationships contained conflicting signals, preserving the complexity discussed in our guide to giraffe genetics.
This extends what a genome comparison can ask. Studies such as Chang Liu and colleagues’ gene-editing investigation test changes in protein instructions for effects on physiology. Petersen’s study examined changes in the placement of larger pieces of DNA. Both are evolutionary variation, but they reveal different aspects of how a genome changes.
The most direct next step would be to test which intact-looking LINE1 and RTE copies can actually generate new insertions. Better complete sequences would help distinguish damaged copies from assembly gaps; functional experiments could distinguish apparent capacity from demonstrated activity. For now, the population comparison has shown why a genome’s most abundant historical residents need not be its most active present-day copiers.
Sources
Malte Petersen, Sven Winter, Raphael Coimbra, Menno J. de Jong, Vladimir V. Kapitonov, Maria A. Nilsson (2021). Population analysis of retrotransposons in giraffe genomes supports RTE decline and widespread LINE1 activity in Giraffidae. Mobile DNA. DOI: 10.1186/s13100-021-00254-y. Read the full paper.
