Department of Environmental Sciences, Zoology, University of Basel, Basel, Switzerland.
Genome Biol Evol. 2023 Jun 1;15(6). doi: 10.1093/gbe/evad111.
The insect order Lepidoptera (butterflies and moths) represents the largest group of organisms with ZW/ZZ sex determination. While the origin of the Z chromosome predates the evolution of the Lepidoptera, the W chromosomes are considered younger, but their origin is debated. To shed light on the origin of the lepidopteran W, we here produce chromosome-level genome assemblies for the butterfly Pieris mannii and compare the sex chromosomes within and between P. mannii and its sister species Pieris rapae. Our analyses clearly indicate a common origin of the W chromosomes of the two Pieris species and reveal similarity between the Z and W in chromosome sequence and structure. This supports the view that the W in these species originates from Z-autosome fusion rather than from a redundant B chromosome. We further demonstrate the extremely rapid evolution of the W relative to the other chromosomes and argue that this may preclude reliable conclusions about the origins of W chromosomes based on comparisons among distantly related Lepidoptera. Finally, we find that sequence similarity between the Z and W chromosomes is greatest toward the chromosome ends, perhaps reflecting selection for the maintenance of recognition sites essential to chromosome segregation. Our study highlights the utility of long-read sequencing technology for illuminating chromosome evolution.
鳞翅目昆虫（蝴蝶和蛾类）代表了具有 ZW/ZZ 性别决定的最大生物类群。虽然 Z 染色体的起源早于鳞翅目昆虫的进化，但 W 染色体被认为较年轻，但它们的起源仍存在争议。为了阐明鳞翅目动物 W 染色体的起源，我们在此为蝴蝶 Pieris mannii 生成了染色体水平的基因组组装，并比较了 P. mannii 及其姐妹种 Pieris rapae 内和种间的性染色体。我们的分析清楚地表明，这两个 Pieris 物种的 W 染色体具有共同的起源，并揭示了 Z 和 W 在染色体序列和结构上的相似性。这支持了这些物种的 W 起源于 Z-常染色体融合而非冗余 B 染色体的观点。我们进一步证明了 W 染色体相对于其他染色体的极其快速进化，并认为这可能排除了基于远缘鳞翅目动物之间的比较来确定 W 染色体起源的可靠结论。最后，我们发现 Z 和 W 染色体之间的序列相似性在染色体末端最大，这可能反映了对维持与染色体分离至关重要的识别位点的选择。我们的研究强调了长读测序技术在阐明染色体进化方面的实用性。
