Sex chromosomes have evolved hundreds of times across the flowering plant tree of life; their recent origins in some members of this clade can shed light on the early consequences of suppressed recombination, a crucial step in sex chromosome evolution. Amborella trichopoda, the sole species of a lineage that is sister to all other extant flowering plants, is dioecious with a young ZW sex determination system. Here we present a haplotype-resolved genome assembly, including highly contiguous assemblies of the Z and W chromosomes. We identify a ~3-megabase sex-determination region (SDR) captured in two strata that includes a ~300-kilobase inversion that is enriched with repetitive sequences and contains a homologue of the Arabidopsis METHYLTHIOADENOSINE NUCLEOSIDASE (MTN1-2) genes, which are known to be involved in fertility. However, the remainder of the SDR does not show patterns typically found in non-recombining SDRs, such as repeat accumulation and gene loss. These findings are consistent with the hypothesis that dioecy is derived in Amborella and the sex chromosome pair has not significantly degenerated.

性染色体在开花植物生命树上进化了数百次;它们最近起源于该分支的一些成员，可以揭示抑制重组的早期后果，这是性染色体进化的关键一步。Amborella trichopoda 是与所有其他现存开花植物相媲美的谱系中的唯一物种，具有年轻的 ZW 性别决定系统。在这里，我们提出了一个单倍型解析的基因组组装，包括 Z 和 W 染色体的高度连续组装。我们鉴定了一个在两个地层中捕获的 ~3 兆碱基性别决定区 （SDR），其中包括一个 ~300 千碱基的倒置，该倒置富含重复序列，并包含拟南芥甲基硫代腺苷核苷酶 （MTN1-2） 基因的同源物，已知参与生育能力。然而，SDR 的其余部分没有显示通常在非重组 SDR 中发现的模式，例如重复积累和基因丢失。这些发现与雌雄异株源自 Amborella 且性染色体对未显着退化的假设一致。