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Difficulty: HardInferences from the Text

When exogenous retroviruses infect germline cells, their integrated proviral genomes can become fixed within host populations as endogenous retroviruses (ERVs), inherited vertically across generations. Over evolutionary epochs, the vast majority of ERV loci succumb to neutral genetic drift, accumulating nonsynonymous mutations and deletions that eliminate viral replication capability. However, genomic analyses reveal that natural selection has occasionally preserved specific viral open reading frames through exaptation—the functional co-optation of viral proteins for host physiology. A prominent example is the syncytin gene family, which encodes envelope glycoproteins repurposed by placental mammals to facilitate cell-cell fusion in the embryonic syncytiotrophoblast layer. Notably, the syncytin genes identified in primates share no orthologous sequence alignment with the syncytin genes found in rodents, demonstrating that independent retroviral insertion events occurred and were co-opted independently in ancestral lineages after the evolutionary divergence of primates and rodents. Additionally, even where the protein-coding sequences of ERVs have degraded entirely into nonfunctional pseudogenes, their flanking long terminal repeats (LTRs)—which historically functioned as viral promoters and enhancers—frequently persist as functional regulatory elements capable of driving host gene expression in tissue-specific contexts.

Based on the passage, which of the following statements are logically supported by the text? (Select all that apply.)

  1. The physiological utility of an endogenous retroviral element to a host organism does not strictly depend on the preservation of intact viral protein-coding sequences.Answer
  2. The common ancestor of primates and rodents did not rely on the specific retroviral envelope genes known as primate syncytins for placental cell-cell fusion.Answer
  3. C
    All mammalian lineages possessing a syncytiotrophoblast layer acquired their cell-cell fusion capabilities exclusively through retroviral envelope exaptation rather than non-viral ancestral mechanisms.
  4. D
    Nonsynonymous mutations occurring within germline retroviral insertions are systematically eliminated by purifying selection unless the viral element immediately confers an adaptive physiological advantage.
  5. E
    Endogenous retroviruses that retain functional promoter activity within their long terminal repeats consistently pose an active threat of generating infectious viral progeny.

Answer

The statements logically supported by the passage are that the physiological utility of an endogenous retroviral element does not strictly depend on intact protein-coding sequences, and that the common ancestor of primates and rodents did not rely on primate syncytins for placental cell-cell fusion.
The text explicitly validates two key deductions: first, that ERV regulatory sequences (LTRs) can provide physiological utility to the host by regulating gene expression even when protein-coding sequences are completely nonfunctional; second, that because primate syncytin insertions took place after the lineage diverged from rodents, the common ancestor existing prior to that divergence did not possess or use primate syncytins.

Step-by-Step Solution

1
Analyze the scope of non-coding ERV functions described in the text.
The final sentence demonstrates that long terminal repeats (LTRs) can drive tissue-specific host gene expression even when viral protein-coding sequences have degraded completely into nonfunctional pseudogenes.
This directly confirms that physiological utility does not require intact protein-coding sequences.
2
Synthesize the chronological timeline of primate and rodent lineage divergence relative to syncytin insertion events.
Because primate syncytins arose from independent retroviral insertions after the divergence from rodents, the ancestral population prior to divergence could not have possessed or utilized primate syncytins.
This establishes that the common ancestor did not utilize primate syncytins for placental fusion.
3
Evaluate the remaining answer choices against explicit qualifiers and scope.
The other choices introduce unsupported universal claims regarding all mammals, reverse the mechanism of neutral drift versus purifying selection, or introduce unstated risks of viral infectivity.
Eliminates all options containing unwarranted logical leaps and external assumptions.

Key Concept

Drawing precise deductive inferences from dense academic prose by tracking causal timelines and scope qualifiers without introducing outside domain knowledge.
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Inferences from the Text

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