Humulus japonicus (currently treated as Humulus scandens): Published Chromosome Numbers
In Humulus japonicus (= Humulus scandens), several chromosome numbers have been reported in the literature, which has led to recurring confusion. However, some values are well established and consistently documented.
✅ Confirmed & Well-Documented Chromosome Numbers
1️⃣ 2n = 16 (♀, XX)
Repeatedly confirmed
Female plants
Standard XX sex chromosome system
Considered the reference value in many floristic and cytological works
2️⃣ 2n = 17 (♂, XY₁Y₂)
Documented in multiple cytogenetic studies
Male plants
Complex sex chromosome system: XY₁Y₂
The additional Y chromosome explains the odd chromosome number
👉 This is one of the most remarkable features:
H. japonicus exhibits a multiple sex chromosome system (XY₁Y₂), unlike Humulus lupulus and Cannabis sativa, which possess a simpler XY system.
3️⃣ Additional Reported Numbers
Further chromosome counts have been described:
2n = 18
2n = 19 (rare; often associated with unstable lines)
2n = 20 (occasionally reported)
>20 (experimental or induced polyploid lines)
These higher numbers are generally linked to:
Aneuploid populations
Experimental cytogenetic work
Rare or unstable lineages
📊 Summary Table
Sex
Chromosome Number
System
Female
2n = 16
XX
Male
2n = 17
XY₁Y₂
Variable
18–20+
Aneuploid / Polyploid
Why “Humulus japonicus” is not the same as Humulus japonicus
Two annual hops – two completely different biological lineages
In modern botany, Humulus japonicus and Humulus scandens are usually treated as the same species.
However, practical trials, morphology, and cytogenetics suggest a different picture.
🌱 Historically, at least two distinct annual Humulus lineages existed
🔹 Line A: the XX/XY lineage (2n = 20)
classic XX/XY sex system
chromosome number 2n = 20
graft-compatible with Cannabis sativa
shows signs of hybridization potential
morphology:
shorter, rounded cotyledons
rounder leaf shape
more compact growth habit
functionally closer to Cannabis
🔹 Line B: the XY1Y2 lineage (2n = 16/17)
complex multiple-Y sex chromosome system
2n = 16 (♀) / 17 (♂)
not graft-compatible with Cannabis
no signs of hybridization
morphology:
very long, narrow cotyledons
strongly segmented, “cannabis-like” leaves
aggressively climbing growth habit
functionally closer to Humulus lupulus
📊 Short comparison of the two annual hops
Trait | XX/XY lineage | XY1Y2 lineage
Chromosomes | 2n = 20 | 2n = 16 / 17
Sex system | XX / XY | XX / XY1Y2
Cotyledons | short, rounded | long, narrow
Basic leaf form | rounded | narrow, strongly lobed
Growth habit | compact | strongly climbing
Grafting onto Cannabis | ✅ yes | ❌ no
Hybridization signs | ✅ yes | ❌ no
Functional proximity | Cannabis | Hop
🧠 Why this matters
These differences are not minor variations — they involve:
chromosome structure
sex determination
developmental biology
functional compatibility
➡️ These are classic species-level boundaries.
The current taxonomic unification is based on simplification —
not on functional biology.
🌿 Conclusion
There were (and are) two biologically distinct annual hops
that were historically grouped under one name.
Anyone working with grafting, hybridization, or breeding notices the difference immediately.