Avian germline-restricted chromosomes are reservoirs for active and expressed long-terminal-repeat retroviruses.

Germline-restricted chromosomes in male house finch (Haemorhous mexicanus) are repeat-rich (63.7%–79.5%) and accumulate intact LTR retroviruses, with higher ERV transcription in testis.

• Why it matters: GRC contents and evolutionary mechanisms in songbirds are poorly understood, especially how they differ from regular chromosomes in retaining and expressing retroviral elements.
• What they did: Comparative long-read genomics on newly assembled male house finch and common rosefinch (Carpodacus erythrinus) GRCs, plus blue tit (Cyanistes caeruleus), assessed repeat content, ERV/LTR accumulation, and transcriptomic ERV expression.
• The result: ERV proliferation peaked during passerine radiation, and in HF testis ERV transcription is significantly higher on GRCs than on regular chromosomes, implying GRCs can serve as reservoirs of active ERVs.