Assortative mating based on heritable body size suggests potential ecological speciation in monarch butterflies Open Access

Abstract

Geographical isolation that separates a species into separate populations can cause the two populations on each side of the physical barrier to evolve independently, as they adapt to different environments and accumulate genetic and phenotypic differences. Consequently, adaptation to different ecological conditions can give rise to divergence in morphology and might result in reproductive isolation. In animals exhibiting courtship tactics, it is well-known that adaptive morphological changes can create reproductive barriers. However, it remains relatively unclear how morphology affects reproductive isolation in species adopting coercive mating strategies. Monarch butterflies employ sexual coercion, and their size varies among different populations. Monarchs originate in North America, where they are migratory, and have dispersed around the world to form non-migratory populations. In general, these non-migratory monarchs have smaller body size. Here, we examine the effects of body size on mating success in the tractable monarch butterfly system. Mating trials were conducted using wild type and size-manipulated monarchs, and the heritability of body size was measured. Our data show that monarchs mate assortatively based on size, and female choice plays a larger than expected role in driving the mating dynamics. In addition, we found that body size is a heritable trait in both migratory Eastern North American monarchs and non-migratory Puerto Rican monarchs. This study sheds light on how morphological adaptations can facilitate speciation in animals that use coercive mating strategies. 

Table of Contents

Introduction ...............................................................................................................................................1

Method .......................................................................................................................................................5

Mating Experiment ...............................................................................................................................5

Experimental manipulation of monarch body size ........................................................................5

Mating assay..............................................................................................................................7

Behavioral analysis for recorded cages.........................................................................................9

Quantification of copulations in spot-checked cages ....................................................................9

Statistical analysis................................................................................................................... 10

Heritability Experiment ....................................................................................................................... 11

Raising Eastern North American monarch butterflies..................................................................11

Raising Puerto Rican and Eastern North American monarchs for the common garden experiment..11

Scanning butterflies ................................................................................................................ 12

Statistical analysis................................................................................................................... 12

Results......................................................................................................................................................13

Mating Experiment............................................................................................................................. 13

Heritability Experiment .......................................................................................................................16

Discussion.................................................................................................................................................19

References ................................................................................................................................................22

About this Honors Thesis

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School	Emory College
Department	Biology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Ecology Biology, General Biology, Evolution and Development
Keyword	heritability speciation reproductive isolation sexual coercion
Committee Chair / Thesis Advisor	de Roode, Jacobus , Emory University
Committee Members	Weinschenk, Matthew , Emory University Beck, Christopher , Emory University

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