Exploitation of an ancestral pheromone biosynthetic pathway contributes to diversification in Heliconius butterflies

Abstract

During courtship, male butterflies of many species produce androconial secretions containing male sex pheromones (MSPs) that communicate species identity and affect female choice. MSPs are thus likely candidates as reproductive barriers, yet their role in speciation remains poorly studied. Although Heliconius butterflies are a model system in speciation, their MSPs have not been investigated from a macroevolutionary perspective. We use GC/MS to characterize male androconial secretions in 33 of the 69 species in the Heliconiini tribe. We found these blends to be species-specific, consistent with a role in reproductive isolation. We detected a burst in blend diversification rate at the most speciose genus, Heliconius ; a consequence of Heliconius and Eueides species using a fatty acid (FA) metabolic pathway to unlock more complex blends than basal Heliconiini species, whose secretions are dominated by plant-like metabolites. A comparison of 10 sister species pairs demonstrates a striking positive correlation between blend dissimilarity and range overlap, consistent with character displacement or reinforcement in sympatry. These results demonstrate for the first time that MSP diversification can promote reproductive isolation across this group of butterflies, showcasing how implementation of an ancestral trait, the co-option of the FA metabolic pathway for pheromone production, can facilitate rapid speciation.

Variation of Clasper Scent Gland Composition of Heliconius Butterflies from a Biodiversity Hotspot

“…Visualisation of the results showed that individuals were mostly grouped by clade, with few deviations (Figure 29) and that most of the variation in CSG composition was explained by species identity (Figure 29), as found in previous studies [15,20,23] . Thus, species identity appears to be a strong predictor of CSG composition previously reported for the male androconial compounds [48] . Nevertheless, some overlap between species occurs.…”

“…Often certain compounds are found in several species, suggesting a common biosynthetic machinery within the heliconiines. Previous results suggest that most CSG compounds are indeed biosynthesised de novo [20,48,55] . The current analyses and previous work [9,15,20,23] indicate that some compounds or groups are particularly important, in addition to those with proven function ( 1 – 5 ).…”

“…Major constituents are the volatile unknown B135 ( U1 ), (3 Z ,6 Z ,9 Z )‐3,6,9‐tricosatriene ( 26 ), (6 Z ,9 Z )‐6,9‐tricosadiene ( 27 ), ( Z )‐9‐tricosene and the fatty acid esters sulcatyl palmitate ( 29 ) and oleate ( 30 ) (Figure 9). Polyunsaturated hydrocarbons 26 and 27 represent typical type II moth pheromone components also found in the androconia of H. demeter [48] . Sulcatyl esters 29 and 30 were first reported from CSGs of Agraulis vanilla e together with sulcatone ( 13 ) as the major volatile.…”

The scent chemistry of butterflies

“…24). 120 H. erato is an exception, lacking any fatty acid metabolites. The androconia contain small amounts of geranylgeranylacetone ( 119 ), 94 but the major component is terminally oxidized 119 conjugated to fatty acids, which is non-volatile.…”

“…The fatty acid metabolic pathway was recruited by more advanced genera to arrive at more complex blends, while basal lineages are dependent on plant metabolites such as 117–120 , or geranylgeranyl acetate ( 123 ) of H. erato and H. himera . 120 The fatty acid derived compounds include mostly compounds similar to type I moth pheromones compounds such as 116 and hexadecanal ( 132 ) of H. aoede , 1-hexadecanol ( 124 ), ( Z )-9-octadecen-1-ol ( 125 ) of H. xanthocles , ( Z )-11-eicosen-1-ol ( 126 ) of H. atthis , ( Z )-13-docoscen-1-ol ( 127 ) of Eueides tales , ( Z )-11-eicosenyl acetate ( 128 ) of E. aliphera , but also alkanes such as heneicosane, tricosane, and ( Z )-9-tricosene ( 129 ) of H. charithonia , as well as (3 Z ,6 Z ,9 Z )-3,6,9-henicosatriene ( 130 ) of H. demeter , a typical type II moth pheromone (Fig. 24).…”