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Investigating the Functional Morphology of Genitalia During Copulation in the Grasshopper Melanoplus Rotundipennis (Scudder, 1878) Via Correlative Microscopy

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Investigating the Functional Morphology of Genitalia During Copulation in the Grasshopper Melanoplus Rotundipennis (Scudder, 1878) Via Correlative Microscopy JOURNAL OF MORPHOLOGY 278:334–359 (2017) Investigating the Functional Morphology of Genitalia During Copulation in the Grasshopper Melanoplus rotundipennis (Scudder, 1878) via Correlative Microscopy Derek A. Woller* and Hojun Song Department of Entomology, Texas A&M University, College Station, Texas ABSTRACT We investigated probable functions of the males, in terms of the number of components interacting genitalic components of a male and a involved in copulation and reproduction (Eberhard, female of the flightless grasshopper species Melanoplus 1985; Arnqvist, 1997; Eberhard, 2010). This rela- rotundipennis (Scudder, 1878) (frozen rapidly during tive complexity has often masked our ability to copulation) via correlative microscopy; in this case, by understand functional genitalic morphology, partic- synergizing micro-computed tomography (micro-CT) with digital single lens reflex camera photography with ularly during the copulation process. This process focal stacking, and scanning electron microscopy. To is difficult to examine intensely due to its often- assign probable functions, we combined imaging results hidden nature (internal components shielded from with observations of live and museum specimens, and view by external components) and because it is function hypotheses from previous studies, the majority often easily interrupted by active observation, but of which focused on museum specimens with few inves- some studies investigating function have manipu- tigating hypotheses in a physical framework of copula- lated genitalia as a way around such issues tion. For both sexes, detailed descriptions are given for (Briceno~ and Eberhard, 2009; Grieshop and Polak, each of the observed genitalic and other reproductive 2012; Dougherty et al., 2015). Adding further com- system components, the majority of which are involved in copulation, and we assigned probable functions to plexity to understanding function, several studies these latter components. The correlative microscopy have demonstrated that individual genitalic compo- approach is effective for examining functional morphol- nents may play different roles during copulation ogy in grasshoppers, so we suggest its use for other and evolve at different rates (Song and Wenzel, animals as well, especially when investigating body 2008; Song and Bucheli, 2010; Rowe and Arnqvist, regions or events that are difficult to access and under- 2011). Furthermore, when studying the evolution of stand otherwise, as shown here with genitalia and cop- genitalia, there has been a continued general bias ulation. J. Morphol. 278:334–359, 2017. VC 2017 Wiley toward examining the morphology of male genitalia Periodicals, Inc. for various reasons, including the difficulties in KEY WORDS: anatomy; insect; Acrididae; sexual selec- examining female genitalic morphology (Eberhard, tion; speciation 1985) (e.g., a common lack of rigidity), assumptions that female components do not appear to vary as much between conspecifics (which may also be true INTRODUCTION in some cases) (Eberhard, 1985; Ah-King et al., The evolution of animal genitalia is an active area of research in evolutionary biology and it is Additional Supporting Information may be found in the online generally accepted that sexual selection is the version of this article. major driving force that explains the observed divergence of genitalic components often found Contract grant sponsors: Orthopterists’ Society Research Grant among closely related species (especially in males (2012, to D.A.W.), and NSF Grant IOS-1253493 and Texas A&M University start-up (to H.S.). and particularly in insects), with these divergen- ces playing a potentially significant role in specia- *Correspondence to: Derek A. Woller, Minnie Belle Heep Center, tion (Eberhard, 1985; Arnqvist et al., 2000; Room 412, Campus MS 2475, College Station, TX 77843-2475. Arnqvist and Rowe, 2002; Hosken and Stockley, E-mail: [email protected] 2004; Ritchie, 2007; Hotzy et al., 2012; Richmond et al., 2012; Simmons, 2013). Insights into the Received 24 August 2016; Revised 18 November 2016; underlying mechanisms of this evolutionary pro- Accepted 3 December 2016. cess, however, are difficult to gain for a number of Published online 23 January 2017 in reasons. For one, insect genitalia are extremely Wiley Online Library (wileyonlinelibrary.com). diverse and often relatively complex, especially in DOI 10.1002/jmor.20642 VC 2017 WILEY PERIODICALS, INC. FUNCTIONAL MORPHOLOGY DURING COPULATION 335 Fig. 1. Melanoplus rotundipennis, abdomina of original male and female specimens frozen in copula compared to their 3D-reconstructions using micro-CT scanning (for all, female is above and male is below): A. left lateral view; B. right lateral view; C. left lateral view of 3D- reconstructions; D. right lateral view of 3D-reconstructions. 2014), and because differences in male components stacking and scanning electron microscopy (SEM), are typically more obvious (Eberhard, 1985; Arnqv- a method known as correlative microscopy (Caplan ist, 1997). However, male and female genitalia have et al., 2011), to investigate the copulation process presumably co-evolved, and, therefore, should be in the context: functional morphology. Compre- studied together. hending the function of morphology, principally We are currently in the midst of an insect (and how the male and female parts interact, is a key other invertebrates) morphology renaissance ever to better understanding genitalia evolution (Kelly since a pioneering study in by Hornschemeyer€ and Moore, 2016). The copulation process in et al. (2002) used micro-computed tomography insects has previously been difficult to study in (micro-CT) to examine beetle morphology. This fine detail because of the cryptic nature of interac- powerful imaging technology is non-invasive and tions between male and female genitalia and the non-destructive and has been used by many to invasive techniques that were necessary for exami- investigate the morphology of insect morphology nation. This frontier is ripe for exploration with in novel ways (e.g., Jongerius and Lentink, 2010; relatively few studies focusing on functional geni- Wipfler et al., 2011; Wojcieszek et al., 2012; talic morphology in insects (e.g., Eberhard and Breeschoten et al., 2013; Faulwetter et al., 2013; Ramirez, 2004; Briceno~ and Eberhard, 2009; Grie- Michalik et al., 2013; Beutel et al., 2014; Greco shop and Polak, 2012; Briceno~ et al., 2016; Rheber- et al., 2014; Simonsen and Kitching, 2014). One of gen et al., 2016), with even fewer utilizing micro- the primary benefits of micro-CT is the ability to CT to do so (e.g., Dougherty et al., 2015; Holwell build three-dimensional (3D) reconstructions of et al., 2015; Mattei et al., 2015; Schmitt and Uhl, any anatomical component included in the high- 2015). In terms of the order Orthoptera, only one resolution scanning process (Fig. 1A–D). For this other study of a similar nature has yet been study, we couple this technology synergistically undertaken (with Metrioptera roeselii (Hagenbach, with two other imaging methods, digital single 1822) (Ensifera: Tettigoniidae) in Wulff et al. lens reflex (DSLR) camera photography with focal (2015), but, to our knowledge, our study is the first Journal of Morphology 336 D.A. WOLLER AND H. SONG (here defined as Alabama, North and South Caroli- na, Georgia, and Florida), and many are endemic to relatively small regions. Externally, all species resemble one another to a strong degree, but it is the male genitalia, primarily the aedeagus, that separate them due to their highly divergent forms (Hubbell, 1932). These collective factors make the Puer Group an excellent system in which to exam- ine the evolution of genitalia. Of all group members, M. rotundipennis was chosen for four reasons, the first being that it is the most widely distributed (in both Florida and southern Georgia), meaning that locating populations, normally quite abundant, was easy and integral to encouraging specimens to mate in the lab within habitat boxes (Fig. 2C). The second is that intraspecific variation in male genitalia, especially in the aedeagi, is negligible (Squitier et al., 1998), http://onlinelibrary.wiley.com/journal/ 10.1002/(ISSN)1097-4687/homepage/ForAuthors. html, which may also be true for female genitalia. Third, the species is one of the largest in the Puer Group with an average length of 15 mm in males and 20 mm in females and the aedeagus is males in also one of the most prominent, making this species one of the easiest to work with overall. Finally, the 58 named genitalic and other reproductive system components across both sexes (33 male, 25 female) of M. rotundipennis can arguably be labeled as the most complex in the Puer Group in terms of number and structural shape based on comparative observa- tions, making this species interesting to study. The general functions of grasshopper genitalia during copulation have been described and specu- lated on by numerous sources (Federov, 1927; Bol- dyrev, 1929; Snodgrass, 1935; Kyl, 1938; Roberts, 1941; Dirsh, 1956; Randell, 1963; Gregory, 1965; Otte, 1970; Pickford and Gillott, 1971; Whitman and Loher, 1984; Snodgrass, 1993; Eades, 2000; Chapman, 2012; Song and Marino-P~ erez, 2013), with the majority focused on museum specimens Fig. 2. Melanoplus rotundipennis,livephotosof:A. male; B. and many biased toward male anatomy. Only five female; C. in copula pair. of those studies (Boldyrev, 1929; Kyl, 1938; Grego- ry, 1965; Pickford and Gillott, 1971; Whitman and Loher, 1984) investigated hypotheses
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