Sedgwick Reserve

01 Jul, 10 AM - 13 Jul, 01 AM

This year I would like to (1) assess cricket density via transects in different habitat types (e.g. more disturbed vs. less disturbed areas), (2) collect animals to send back to the University of Nebraska-Lincoln for replenishing the laboratory stock with new genetic diversity, (3) collect animals to sacrifice, half for making several body measurements, including gonad mass and flight muscle color, and half for freezing and bringing back to UNL for measurements of total body lipid as well as C/N ratios, and (4) conduct an experiment measuring ability of males of different wing morphs to attract females. A detailed proposal is attached: The evolution of life history traits is often constrained, and an evolutionary increase in one trait can lead to an evolutionary decrease in correlated traits (Stearns 1976, Partridge & Harvey 1988, Partridge & Sibley 1991, Roff 1992). Such trade-offs may arise due to resource limitations, and organisms may have to ?choose? how to allocate limited resources to maximize lifetime fitness (Schlichting & Pigliucci 1998). The resulting negative associations between traits, such as current and future reproduction (Partridge & Farquhar 1981), age and size at sexual maturation (Reznick & Endler 1982), immunity and reproduction (Zuk & Stoehr 2002) and flight capability and reproduction (Zera et al. 1998) have been examined in a variety of organisms. Wing-polymorphism in insects is one system which exemplifies the evolutionary consequences of such trade-offs (Roff 1986, Dingle 1996, Zera & Harshman 2001). In these insects, flight capability trades off with reproductive traits between morphs. There is often a reproductive morph (flightless, short-winged (SW) individuals) and a flight-capable morph (long-winged (LW) individuals; Zera & Harshman 2001). In order to fly, LW individuals have to maintain energetically expensive flight muscles, and this results in lower reproductive success (Mole & Zera 1993, Crnokrak & Roff 1995). More recently, it has been found that LW individuals can breakdown their flight muscles, becoming flightless, and increase reproductive investment (Zera et al. 1997). In an exciting new development, in the course of my work with G. lineaticeps, I have found a new morph of SW cricket ? a SW with what appear to be flight muscles resembling those of flight capable LW individuals (Mitra unpublished data). Preliminary work, has shown that individuals of both wing morphs (LW & SW) with flight muscles pay a reproductive cost in comparison to individuals without; females have significantly smaller ovaries and are less phonotactic, and males have significantly smaller accessory glands and spend less time singing (Mitra unpublished data). While I did not found significant differences in risk aversion (an indirect measure of predation risk), initial work suggests SW individuals are better at surviving bacterial infection (Mitra unpublished data). I am continuing work to examine whether there are morph-specific differences in jumping ability (anti-predator response), male-male competition and female choosiness. In addition, previous work has shown that morph determination (SW or LW) is affected by both genes and the environment; it is a threshold trait, polyphenetic within a single individual, but with genetic variation in the threshold (Zera & Denno 1997). Under lab conditions, we know that density, food quality, temperature and photoperiod prior to sexual maturity affect morph expression (McFarlane 1962, Masaki & Oyama 1963, Saeki 1966). Environmental conditions affecting flight-muscle breakdown in either morph are as yet unknown. Many details of this polymorphism have been studied in many labs, often with cricket lines artificially selected for morph. However, we know surprisingly little about this polymorphism in the field. In order to formulate biologically relevant hypotheses about the evolution of wing-polymorphism, including why some SW individuals pay the cost of muscles that they appear to have no use for, we need to study these animals in the field. I am applying for funding to conduct a field survey of and a field experiment with my study species the variable field cricket, G. lineaticeps. The questions I hope to answer include, (1) is there variation in the distribution of different morphs in space (different habitat types), time (different times of the breeding season), or with varying densities of individuals; (2) do we see similar reproductive costs of flight-muscles in the field as we did in the lab (variation in ovary and male accessory gland size); (3) do males without flight muscles attract more females, and (4) is there variation between morphs in average age post sexual maturity in the field (are individuals with flight-muscles delaying reproduction but living longer)? Field Survey: Methods, Study Species and Study Area: Gryllus lineaticeps are active all year, but concentrate their breeding activity in the summer months between June and August. I would like to examine a total of five populations in southern California, spanning a variety of habitat types (agricultural, grassland, and coastal scrub). My sites will be spread through south-western California from Fresno, to Monterey through to Thousand Oaks. One of my advisors, Dr. William E. Wagner Jr., has collected in this area before, and I am planning on staying at and surveying around several of University of California?s Natural Reserves (including Hastings and Sedgwick). I am planning on spending one week or more each site. At each site I plan on (1) getting an estimate of field density of individuals by intensively searching along transects (3 per site, 500m long and 1m wide), and (2) collecting crickets by searching (I have collected via this method before). The day after I collect animals, I will (1) record how many individuals of each morph I catch (muscle status can be determined without dissecting by examining colour of the muscle patch behind the hind leg), (2) weigh all individuals and measure body size (commonly measured as pronotum width), (3) dissect some individuals and weigh female ovary and male accessory gland mass and (3) freeze the hind legs for aging later (crickets can be aged post sexual maturity by counting cuticle rings in the leg). The remainder of the crickets will be frozen and brought back to the lab at UNL; these will be analyzed for total lipid content to provide us with an estimate of habitat quality (diet quality of the crickets) for each site . Results and Interpretation: From the results of this study, I will be able to assess what environmental factors are important for morph distributions: e.g. habitat quality (assessed via lipid analyses of the crickets) and density. I will also be able to assess if morph specific differences in ovary and male accessory gland mass seen in the lab are also seen in the field. Lastly, by aging field caught crickets, I will be able to assess whether there is morph specific variation in average age in the field. Variation among males in ability to attract females Methods, Study Species and Study Area: This experiment, examining male ability to attract mates, will be run over two weeks at Sedgwick Reserve, of the University of California Natural Reserve System, in the Santa Ynez valley north-east of Santa Barbara, California. In this species males produce calling song from the entrances of burrows to attract females. When females approach, males switch to producing courtship song; if a female chooses to mate with the male she will enter the burrow with him. Several lab studies have shown that amount of time spent calling, as well as variation in male song characteristics, affect a male?s ability to attract females (e.g. Hedrick 1986, Wagner & Reiser 2000) However, there is no field evidence that variation in male song type is related to number of females he attracts. Therefore, this experiment is aimed at examining the effect of (1) wing-polymorphism and (2) male calling behaviour on how many females a male attracts. I will find singing males by searching, record their song using a microphone and tape recorder, and will catch and ascertain wing morph of each individual before the start of the trials. Songs will be analyzed later in the lab for song characteristics such as chirp rate, chirp duration, dominant frequency, etc. After males are recorded, a cage (a cylinder made of window netting, 10cm in diameter and height, open at the bottom) will be placed over their burrow. A circle cut from clear plastic sheeting (50 cm in diameter, with a hole 10 cm in diameter in the middle), coated with Tanglefoot (a trapping adhesive used for catching insect pests) will be placed around the cage. From trial-runs last summer we know that Tanglefoot is effective in trapping insects venturing on it, that the cage keeps the male from getting stuck himself, and that the male will continue singing inside the cage as long as he is not moved from his burrow. The setup will be completed soon after sundown, and the trial will be run all night. I will return to each site every 30 minutes until sunrise, to assess proportion of time males spend singing. At sunrise, I will return to each site to count number of females trapped in the Tanglefoot, breakdown the set-up and remove the cage. If males can be recaptured in the morning, I will remove a hind leg to use for aging later. Males will be marked with a paint spot on their pronotum to ensure the same male is not re-used during the study. Depending upon how many males I can record and catch, I plan on running between 10 and 20 trials per night. Results and Interpretation: From the results of this experiment, I will be able to assess whether (1) wing morph, (2) male song characteristics, or (3) amount of time a male spent singing, affect number of females a male attracts. I expect that if song characteristics are important to female assessment of mates, males with high chirp rates and long chirp durations (these males are preferred by females, and mating with such males has been shown to increase female fecundity and lifespan, in the lab; Wagner et al. 2001, Wagner & Harper 2003) should attract more females. If display time is important, males who sing more will attract more females. And finally, as previous work has shown that in the lab males with flight muscles sing less, I expect them to attract fewer females than males without flight muscles. Conclusions The survey and the experiment described above will provide much needed ecological context to a lot of current theory on the evolution of wing-polymorphism. It will be the first study to examine whether factor such as density, habitat quality or time of breeding season affect morph determination in the field. In addition, assessing average age of individuals post sexual maturity and reconfirming my lab findings of a reproductive cost of flight muscles with field individuals will not only help me develop further hypotheses in my dissertational research, but also help me parameterize a stochastic dynamic model examining this flight-reproduction life history trade-off. Lastly, as male reproductive success is often difficult to measure directly, aspects of male song, such as display time and song characteristics have often been used as an indirect measure of a male's reproductive success. By examining how these factors actually affect male ability to attract females, we can re-examine the validity of using such indirect measures of male reproductive success. If all goes according to plan, by the end of the summer, I should be able to fill several important gaps in our current knowledge of this much-studied system.

Visit #15183 @Sedgwick Reserve

Approved

Under Project # 7182 | Research

Costs of phenotypic plasticity in the wing-polymorphic field cricket, Gryllus lineaticeps

graduate_student - University of Nebraska

Reservation Members(s)

Chandreyee Mitra Jul 1 - 13, 2008 (13 days)
Chandreyee Mitra Jul 1 - 13, 2008 (13 days)

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Studio Apartment 2 Jul 1 - 13, 2008