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Ageing is the progressive decline in physiological performance with age, which is almost universal amongst multicellular organisms. While understanding ageing is an important aim in biological research, our current understanding of how and why we age is incomplete. In this thesis, I examine how sexual selection affects the evolution and mechanistic basis of ageing. I then explore how diet affects lifespan and reproduction in either sex. Finally, I test the hypotheses that oxidative stress, which occurs when cellular levels of Reactive Oxygen Species exceed circulating antioxidant defences causes ageing (i.e. the free radical theory of ageing) and/or constrains life-history strategies. To ask these questions, I employ quantitative genetics in decorated crickets Gryllodes sigillatus to examine the genetic co(variance) of ageing, lifespan, reproductive effort, oxidative damage and antioxidant protection. In the Australian field cricket, Teleogryllus commodus, I apply the geometric framework of nutrition to examine how lifespan, reproductive effort, oxidative damage and antioxidant capacity respond to dietary manipulation. In G. sigillatus, I found that sexual selection caused divergent strategies of age-dependent reproductive effort across the sexes and that this, in turn, promoted different rates of ageing in males and females. I found a trade-off between early reproductive effort and ageing rate in both sexes, although this trade-off was more pronounced in females (Chapter 3). I then explored the mechanistic basis of these sex-specific life-history strategies and, in support of the free radical theory of ageing, I found that oxidative damage was greatest in the shortest lived sex (females) and was negatively genetically correlated with lifespan. Additionally, oxidative damage was a cost of female reproductive effort that accelerated ageing, showing that oxidative stress may mediate sex-specific life-history strategies in decorated crickets (Chapter 4). If sexual selection affects reproduction and lifespan it should promote sex-specific life-history responses to dietary manipulation. In Australian black field crickets Teleogryllus commodus, I found that males and females have distinct dietary optima for lifespan and reproductive effort and that diet mediated a trade-off between these traits. I found that mating affected responses to dietary manipulation and caused sexual dimorphism in dietary intake under choice (Chapter 5). However, oxidative stress did not explain these life-history responses to dietary manipulation across the sexes (Chapter 6): although oxidative damage was greatest in the shortest lived sex (i.e. females), diets that extended lifespan did not reduce oxidative damage. My thesis illustrates the importance of considering sexual selection when considering the evolution and mechanistic basis of ageing. It offers equivocal support for the free radical theory of ageing but shows that oxidative stress may help underpin sex-specific life-history strategies. However, my results highlight that unravelling the relationship between oxidative stress and life-history strategies across the sexes will be a very difficult task.

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The goal of this project was to design networked arrays of cricket biosensors capable of localizing sources such as footsteps within dangerous environments, with a possible application to earthquake detection. We utilize the cricket's natural ability to localize low frequency (5 Hz - 600 Hz) acoustic sources using hair-covered appendages called cerci. Whereas previous investigations explored crickets' neurological response to near field flows generated by single frequency steady-state sounds, we investigated the effects of transient waveforms, which better represent real world stimuli, and to which the cercal system appears to be most reactive. Extracellular recording electrodes are permanently implanted into a cricket's ventral nerve cord to record the action potentials emanating from the cerci. In order to calibrate this system, we attempt to find the relationships between the frequency and direction of acoustic stimuli and the neurological responses known as spike trains, which they elicit. The degree of habituation to repeated signals that exists in most neurological systems was also experimentally measured. We process the signals to estimate frequency and directionality of near field acoustic sources. The design goal is a bionic cricket-computer system design capable of localizing low frequency near field acoustic signals while going about its natural activities such as locomotion.

Crickets provide a useful model system to study how animals analyze sound frequency. While much is known about how sound frequency is represented by central neurons and in behavior, little is yet known about auditory receptor neurons. I investigated physiological and anatomical properties of auditory receptor fibers (ARFs) and functional organization of their axon terminals, using single-unit recording and staining techniques. Behavioral experiments suggest that crickets are sensitive to two broad frequency ranges, centered at 4--5 kHz for acoustic communication and at 25--50 kHz for predator detection. However, cricket ARFs fall into three distinct populations, based on characteristic frequency (CF; low frequency, ∼3--5.5 kHz; mid frequency, 9--12 kHz; ultrasound, ≥18 kHz). One striking characteristic of single ARFs is the occurrence of multiple sensitivity peaks at different frequencies, which implies that the wide audible range of crickets is mediated by these multiple sensitivity peaks, even though CFs of ARFs are clustered at the three small ranges. To understand how populations of ARFs code sound intensity, level-response functions are examined. Physiological parameters derived from level-response functions are diverse, and are systematically related to threshold within each population. Low-frequency ARFs comprise two distinct anatomical types, based on the distributions of axon terminals, which also differ physiologically. Thus, based on CF and anatomy, cricket ARFs can be classified into four distinct populations. To understand how information flows from peripheral to central neurons, the positions of varicosities, i.e. output sites, of ARF axon terminals are mapped on a two-dimensional coordinate system. In crickets, the ARF axon terminals are functionally organized with respect to frequency and intensity. Anatomical organization with respect to threshold is related to physiological organization, which may reduce non-linear effects in postsynaptic

The evolutionary importance of genetic constraints has always been recognized by biologists, but very little data is available to quantitatively assess the role of constraints in shaping the biology of organisms. The field of quantitative genetics provides the tools necessary to study evolutionary constraints, mainly through the estimation of the matrix of additive genetic variance and covariance (the G matrix). The main goals of this Ph.D. dissertation were to study the persistence of constraints across environments and across species, to explore the consequences of constraints on species divergence, and to try to understand some morphological and life history characteristics of field crickets in light of genetic variation. Populations of seven wing-dimorphic cricket species from the genera Gryllus and Teleogryllus were sampled from the wild and reared in the laboratory. Using multiple statistical approaches to the comparison of G matrices, results revealed little variation in G matrices across species. Moreover, the relatively small effect of rearing environment and of the two wing morphologies on G were shown to be of the same magnitude as variation between species, therefore confirming the general constancy of genetic constraints through evolutionary time scales. Mean trait values, selection regimes and phylogenetic distances were all shown not to be predictors of G matrix variation. In agreement with the constraint hypothesis of quantitative genetic theory, morphological divergence between species was shown to be predictable from a reconstructed ancestral G matrix. In addition, information on genetic variation was used to explain various patterns relating to size, ovipositor length, wing morphology and diapause occurrence in field crickets. Overall, we suggest that genetic constraints, as described by quantitative genetics, have played a major role in shaping the observed biological diversity of field cricket species, a conclusion tha

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Thesis (M.A.) in Mathematics--University of Maine, 2004. / Includes vita. Includes bibliographical references (leaves 40-42).

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